lategame balance . are my concerns true ?

[Tuidjy]

You are worrying for nothing. Hordes of vampires are available only in late
game, and there are plenty of strategies that are at least that good. I have
to admit that I have never faced vampire hordes, but I have messed up devil
hordes simply by teleporting three independant sorceresses on top of Cohen's
main spawn point, and casting wrathful skies and enough horrors to keep the
devils from sprinting to the casters, and then reteating. (I brought a SoS.)
Yes, vampires are immortal, and yes, they will be back, but they are nothing to
write home about. If anything, they will rout much more easily that the devils.

Furthermore, I have had three supercombatants (two arch devils and a heliophagus)
run into a small pack of vampires (30-40) and demolish them... I was surprised,
because my guys were not decked out for undead hunting. But if I had the
chance to properly outfit them (fire shield + fatigue regeneration) I would
bet on five SCs against 500 vampires. (Storm is a must, though) But frankly, by
the time you have hundreds of vampires, your enemy will have something else.

As for domain pushes, you do not need stealthy priests. You need to take
territory, and convert it.