I'd ask you to wait until the vampire counts have their re-release in a week to get anything else. If you want to get anything you might want to check out the new army book and try to form strategies and the composition of your army off of that. Also i'd hold off from getting any more banshees as their rules have been reworked and i'd also hold off on the black knights as they're making a much cheaper and better looking plastic set for them. I really wish i could advise you on what to do but considering their army book is changing i think we're all going to be a bit unsure of what to do with the army for at least a few months.
Luckily for you on the vampire end i hear that they'll have a sort of master necromancer or similar that you can have as lord choice for your general. Supposedly you'll be able to have an entire army without vampires. I am unsure if you mean no vampires as in heroes and lords or whether this means no vampire cavalry, monstrous creatures, chariots and monstrous infantry and pretty much everything with the vampire rule.
You chose a good army i'll give you that. I have a skaven army but VC as well as a few other fantasy armies seem pretty fun to me. The big thing in the previous and possibly the current VC army you have to realize is that you're unbreakable but slowly crumble when you lose or your general dies (a very bad thing btw). Keep your general as far away from combat as you can most of the time as his death ends in your army crumbling slowly. I don't think it's as bad as it used to be with the new army book but you still should watch out. Next magic is good but it's mostly about buffing, hexing and raising undead. There's one magic missile attack and it's alright but not fantastic. You also need to remember vampire counts have almost no or no shooting. The new book might slightly change this but only very slightly. Really the point of vampire counts is raising more undead than the enemy can handle and being able to raise your dead back up. Items and banners are usually really good and you'll find a good amount of worthwhile items even in the current VC army book before the new one is released. 8th edition tends to throw out a lot of stuff but to be fair a lot of the old stuff in many books that got thrown out were garbage that nobody took anyway. It did give each race some variation though. With vampire counts a big deal is you have some of the only ethereal units in the game which can only be harmed by magical weapons, attacks, effects. If somebody isn't ready for this and your unit is very killy then you'll reap a bloody toll but even if their magic is non-existant (dwarfs) they still have items and can beat you with combat resolution if you're not careful. Vampire counts also supposedly have the best knights in the game (blood knights or at least they are until the new release). So yeah usually to win you'll want to overwhelm the enemy with undead, have super killy powerful units or use ethereal units which can't be touched in most cases and you can buff yourself with magic spells and good items which the VC army has plenty of. As far as other strategies go i can't really say as the new book is something i haven't seen before.