In in the end I started already the VC campaign, and in fact I played a lot of hours. Some notes:
-I took the mage instead of Manfred. The idea was to abuse his -15% reduction cost in Raise Dead (which is accumulative with another -7% trait you win after raising undead a few times and another -15% from one of the end skill trees). In fact for most of the campaign I've been raising dead, not using recruitment buildings. I have a pair of big battle points where in fact I can recruit some end-game units.
-I did a very good early game, I rushed and in just a few turns I defeated the two rival VC and had the 3 entire provinces.
-I've been using, and in good effect, armies composed of 3/4 cheap skellies/zombies and 1/4 of cavalry/monsters. In a way these cheap armies were needed because my economy sucked (har har suck, vampires)
-The exception are the sieges, which with this kind of army they are horrible. Cheap units are melted in the tough fight for the walls and cavalry can't flank. In fact I had one battle which I played directly and had 3 full armies vs 1 army + garrison in a city with upgraded walls and damn, I was utterly defeated. In fact I won that one by using the autobattle system, which clearly isn't accurate and let me win by sheer number of forces.
-In the mid game I found problems with lack of vampiric population, and therefore of public order. This slowed me greatly, the campaign would have gone in another direction without this problem. In fact I suppose VC are balanced because of that, they would be easy-peasy otherwise.
-In this campaign I barely played direct battles. Not by conscious choice, but because the battles happened to be mainly one sided affairs (not interesting then) or sieges (boring grind) or both. Mostly, I have been the aggressor and did 'blitzkrieg' on small nations, reaching their settlement and siege them before they could answer.
-Without a special long-term plan, I just attacked what it seemed my weakest neighbor, once at a time. This kind of strategy let me expand to the North, and then to the West, and then finally to the central Imperial provinces. Which it may have been a mistake. Because before I could take the Empire the Chaos horde appeared, and who is the dude at the North, taking all the brunt of their attacks, without no allies? Yep, that's me.
-My current status (lol):
I don't know if I should retreat and wait until they make the mistake of dividing up their armies, or try to hold in the city siege. I could even try to retreat to the South, sacking everything, and with that extra money hire even more armies (I have 5 right now) to finally beat Chaos.