lategame balance . are my concerns true ?

[Jack Simth]

There are a few mitigating things:
One you briefly mentioned: Dominion.
If I'm defending from your attack, it is on turf I hold; with good use of preaching, that pretty much means that unless I just recently conqured the province, it is likely in my dominion. While you are attacking me, chances are it is in my dominion, and any casulties I inflict on your forces are rather permanent.

Another thing to consider is the low morale of vampire thralls. If I have a fair number of tough lifeless troops gaurding several priests, with the priests scripted to spam banish (especially if I include the staff of storms thing to slow the vampire attack), many of your vampire brigades will break before reaching my priests and their bodygaurds, and so never reach them at all.

Another thing to consider is the chance that I have similar recourses at my disposal. Suppose I've done the clamhoarding as well as you have, but instead of wishing for blood slaves, I have been wishing for Dominion, or money, or whatever.

Another thing to consider is that vampires are undead. If I happen to have the spell "Undead Mastery" at the disposal of my highly equipped SC (or a lot of mages with the spell Control The Dead (or whatever it's called)), I can harvest a fair number of those vampires from you, very effectively eliminating them from your grasp regardless of whose dominion we are fighting in. With MR penetration items, I may even be able to arrange to steal more than half of your army out from under you in any given encounter. Wouldn't you just love to face that undead horde you took so much time to build, with pretty much only troops that are vulnerable to the same attack the current leader of said horde can harvest to add to his army?

I'm not saying any of these are foolproof counters, or even that I could properly beat such a strategy for certain, just that it isn't as invulnerable a strategy as you might think.