Muktol

Sorry to say that this doesn't work:
Lets say you've played a [[Lightning Bolt]], copied it with a copied it with a [[Twincast]] and want to use [[Ral, Storm Conduit]]'s -2 ability on [[Twincast]].

From the complete rulebook (http://media.wizards.com/2018/downloads/MagicCompRules%2020180713.pdf):
306.5d Each planeswalker has a number of loyalty abilities, which are activated abilities with loyalty symbols in their costs. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if none of that permanent’s loyalty abilities have been activated that turn.
So you can only use planeswalker ability's when the stack is empty (like sorcery's).

Lets pretend for a moment that this would work nevertheless. The stack will look like this now:
Lets say you cast a [[Lightning Bolt]] and copy it with a [[Twincast]].
Stack (Top Down): [[Twincast]] -> [[Lightning Bolt]]
Now you use [[Ral, Storm Conduit]] ability to create a copy of [[Twincast]] and target the initial Twincast with it.
Stack: [[Twincast]] (Copy) -> [[Twincast]] (Original) -> [[Lightning Bolt]]
The copy resolves, targeting the original, creating a new copy
Stack: [[Twincast]] (Copy2) -> [[Twincast]] (Original) -> [[Lightning Bolt]]
The copy resolves again (and so on), targeting the original, creating a new copy
Stack: [[Twincast]] (Copy3) -> [[Twincast]] (Original) -> [[Lightning Bolt]]
To end this loop you have to target the innitial spell, [[Lightning Bolt]] in this case, and end up with basically 3 Bolts in the end.
Stack: [[Twincast]] (CopyX targeting Bolt now) -> [[Twincast]] (Original, targeting Bolt) -> [[Lightning Bolt]]

The last problem here is that instant/ sorcery's that are copied aren't cast and thus don't count toward storm counter or activate ability's of creatures like [[Guttersnipe]]:
706.10. To copy a spell, activated ability, or triggered ability means to put a copy of it onto the stack; a copy of a spell isn't cast and a copy of an activated ability isn't activated.

Greetings
Muktol

[[Ghostly Flicker]] and [[Conjurer's Closet]] come to my mind. If you want to add a Planeswalker then [[Venser, the Sojourner]] could be worth a look.

Adding a second colour is doable with costs per card from .15$ ([[Orzhov Guildgate]], [[Forsaken Sanctuary]]). Yes it slows you down a little bit, but that shouldn't hurt too much in standard.
Mono-coloured decks have their place and usage but from my experience the standard to built are decks that use 2 colours: The mana-base isn't that complicated and you get access to a second colour that allows you to correct/ improve the weaknesses that a single colour might have.

Personally I would add white as second colour to get access to some good cards:
[[Bishop's Soldier]], [[Famished Paladin]], [[Legion Lieutenant]], [[Martyr of Dusk]], [[Pitiless Pontiff]], [[Skymarcher Aspirant]], [[Vizkopa Vampire]].

Depends an what you have played before. Legacy can be a lot faster and more complex to play, while Standard is normally slower than modern.
Something that I just recognised is that you have [[Dark Prophecy]] and [[Altar's Reap]] for card-draw. I would swap [[Altar's Reap]] to get some removal like [[Tragic Slip]], [[Doom Blade]] or [[Bone Splinters]]

What about [[Risk Factor]] to ensure that you don't run out of spells to throw at your opponent?

A great card when building up devotion and using sacrifice mechanics would be [[Geralf's Messenger]], bringing 3 devotion and the undying mechanic into the deck. [[Gatekeeper of Malakir]] or [[Skinrender]] could be used as alternative to [[Plaguecrafter]] while [[Gifted Aetherborn]]/ [[Vampire Nighthawk]] help you to stay alive while bringing devotion.
[[Undying Evil]] could be an interesting way to re-use your Merchant without having to pay for him again. Sacrifice/ block with him, use the spell on him and voila...re-enteing. [[Lord of the Undead]] could be another way to re-use him and even make him tougher.

Further suggestions:
[[Vampire Hexmage]], [[Dread Shade]], [[Phyrexian Etchings]], [[Phyrexian Obliterator]], [[Creakwood Liege]]

Greetings
Muktol

[[Auramancer]] doesn't fit to [[Mirko Vosk, Mind Drinker]]'s colour identity.

The advantage is that yo have a board-swipe, and the only thing you can loose are some tokens. Another big adavnateg is that his ability ignores hexproof and shroud, the only thing he can't kill are planeswalkers. And if you don't need him anymore because your opponent's creatures did bite the dust you can sacrifice him into his own triggered ability.

Personally I would ry to gut back a little bit on creatures and add some support. Here are some ideas:
Artifacts: Champion's Helm, [[Dragon Throne of Tarkir]], Illusionist's Bracers, Konda's Banner, [[Lightning Greaves]], Magewright's Stone, [[Mask of Avacyn]], [[Swiftfoot Boots]], [[Thornbite Staff]], [[Thousand-Year Elixir]], [[Throne of the God-Pharaoh]]
Enchatntments: [[Blood Moon]], [[Boggart Shenanigans]], [[Fervor]], [[Quest for the Goblin Lord]], [[Shared Animosity]], [[Warstorm Surge]]
If you want to replace some or your creatures I can reccomend [[Bloodmark Mentor]], [[Caterwauling Boggart]], [[Goblin Assassin]] (A most underrated guy in my opinion), [[Goblin Piledriver]], [[Legion Loyalist]], [[Purphoros, God of the Forge]],

Greetings
Muktol

[[Elite Inquisitor]] is a 2/2 with first strike and vigilance, [[Kazandu Blademaster]] enters as a 2/2 with first strike and vigilance. The big difference between both is that [[Elite Inquisitor]] has protection form 3 tribals, where 2 of them will see little action, while [[Kazandu Blademaster]] has the chance to get bigger when you play another Blademaster. Combined with some card-advantage the chances are high that you will be able to play another Blademaster while one is on the field and pump one to a 3/3. Personally I like the chance of making my creature stronger better than the protection from 3 tribals.

[[Kazandu Blademaster]] instead of [[Elite Inquisitor]]
[[Mentor of the Meek]] is a good carddraw-engine, especially when combined with [[Thraben Doomsayer]]

Greetings
Muktol

If you don't want any comments then having no description is fine (at least for me, I simply close such decks as fast as I opened them). But if you want feedback, and that's what I assumed (and wrote: "...I want to have other people look at my decks and leave constructive comments") in this description, them at least some information should be given.

What to remove depends on how you want the deck to behave. Fast? Controlish and mid-game? Unpredictable? I would suggest to make a proper deck-description to give us some informations.

Greetings
Muktol

[[Charge]] vs. [[Pride of Conquerors]] is a tough one for me. On the one hand the cities blessing should be achievable which makes it better then [[Charge]], on the other hand [[Charge]] costs 1 mana less which can also be important.
I would use [[March of the multitudes]] as 1-off since the mana-costs are rather high and you don't want to draw it early on.
[[Venerated Loxodon]] is a good card I'm not totally convinced of. It reminds me on [[Urge to Feed]] but vampire decks normally have a ton of removal to keep their opponent down and thus minimize the chance of a counter-attack.

I added it instead of [[Sprout Swarm]]. [[Saproling Migration]] lowers the mana-curve and is useful thanks to the kicker ability later on.

Problem with [[Elvish Promenade]] is that it will push the deck over the already exceeded cash limit (14-16$). I will give [[Gilt-Leaf Ambush]] a try tough it might be too complicated to understand for a beginner.

*Edit: Just swapped [[Llanowar Elves]] for [[Elvish Mystic]]. Does the same but the average price is 1.11$ for the elves vs. 0.22$ for the Mystic, guess the Elves are played by too many people.

Any ideas of what to replace it with? I planned it with [[Immaculate Magistrate]] in mind as creature that can strengthen others but the Magistrate can be removed rather easily.

Two rectangular bracket's in front and behind the card-name: [[CARDNAME]]. Cards with an apostrophe (') in it's name can't be linked at the moment and the linking also only works in comments.

Yes, but with 1$ as average price it will push the price of the deck way above the desired 14-16$.