The Partnership

As promised, I've looked at your cardlist, some interesting things I've taken a note of, the horn was a mention that is indeed worth the testing, anointed procession being the second I've too taken note. Both these cards I was well aware have made their way as of recently, the Banner being the harder to find due to being limited in the recent Commander decks, but I should be able to get it without much struggle.

Right, now on to the meat of things, you've already have some of the cards I'd make mention of in your sideboard, and I can't stress enough how important the Immortal sun is. Its a deck draw engine without second, definitely the card that made me run it over the Banner. You can run both, I just found myself not needing it due to how I run my deck.

Now off the other cards that are of extreme importance:

Sun Ring. that's two colorless mana that you won't have to worry about getting. Overall Sun ring is ran on every single Commander deck bar none, and there's a good reason for it. Run against someone with a Sun ring on turn one and you'll soon know what you're up against, have it land turns later and its still providing resources for turns on end.
Skullclamp. Draw 2 for free, remember when I said I barely need card draw? Well, that's the magic of Skullclamp, you have an infinite engine for card draw until you land your +1/+1 engines, at which point you should be stocked enough to have things run wild, have it land turns later and you're still having card draw, trust me, board wipes are everywhere, 1 mana for two cards? Sign me in, it keeps on happening.

Those two are of utmost importance bar none, everything else I add is subject to how ones playstyle is. Most of which you can find on my own decklist, so I'd refrain from flooding this answer with them.

Some of which are:

Sanguine Bond. You'll be getting life for days, doubling the amounts of damage one creature does, more so when you can even redirect it, is a big deal, make them unblockable and you're not going anywhere, your opponents on the other hand, if they let you set it up, one single creature will make everyone's life tumble down on a very aggressive pacing, more so because vampires are stacked on the thing, its very easy to let it become out of hand.
Infinite combo with Exquisite blood.

Kalitas. Board wipes, they are everywhere, you're against multiplayer commander and one lands? rejoice, you have a army that they can't deal with without a 2nd of those things, oh and you may have yet, or may have already faced against opponents that are packed with graveyard infinite combos / Sorcery-Instant grave digging /Graveborn decks, yeah, none of that. You're getting all of the rewards while they'll be struggling to even have one board wipe land without repercussion.

Indulgent Aristocrat. Sacrifice one token for +1/+1 on an army. Attack, have opponents assign blockers, set yourself up, sacrifice creatures, kill them.
A creature got exiled? Answer, you've denied an exile and got your creatures pumped. Its a Swiss army knive, and for good reason.

Last suggestions come from me not having played the deck you've listed as is now, but at glance, it looks too piled up with creatures that don't necessarily synergise best with one another, but I'll leave that to you since I'm not running them and thus do not know what results they may possess, however mana curve seems to suffer as a result, and albeit you've got some card draw, it seems that you're giving yourself waay too many requirements for yourself to draw.
Case in point: Skeletal scrying requires Card exile from your own deck, yes you do have instants that you're okay with losing, but until you've set yourself up, it may be a dead card in your hand. Banner, you need the Vampires to have the card draw, happen. Yet you don't take advantage of Eminence from tokens that grantee you free draws with Skullclamp. Not too sure if this was intentional, but I'd personally reconsider some of the options you've layed out.

Aigh, I could go on but I think I've already left a few suggestions that may leave you considering a few options.
Was thankful for yet another interpretation of the Tribal Vampire themed deck and thankful for having you give your own suggestions, hopefully I too have somehow inspired you to fine-tune your deck.

Cheers!