When the vampires pour out of the sewers and all of Razmir's garrisons are assailed at once, they do not, in one sense, react as badly as you would expect. They know they are garrisons, alone in occupied territory and outnumbered; they know perfectly well that there are powerful undead in Ustalav; they know perfectly well that everyone in the country hates them. They have fortified citadels to retreat to, warded against undead; they travel in groups, they wear armor (those of them who can wear armor) at essentially all times.
That just is not, really, enough. There are citadels, the citadels have garrisons, the troops are in good order, there are silvered swords and arrows, scrolls and wands prepared with Scorching Ray spells with sufficient power to blast vulnerable undead into dust -
- But vampires are immortal. Luvick predates the Whispering Tyrant. And that is a millennium to stockpile magical items with which to equip his army.
Also, vampires are, frankly, ridiculously powerful. A vampire of mediocre experience, who has trained his physical abilities to the utter limit of what mortal skill can muster without risk to life or unlife, is a warrior better than nearly all of Razmir's shackleborn, proof against all weapons but enchanted silver (mundane silver won't work!), resistant to magic, constantly healing, and comes with three vampire spawn almost as deadly as himself and choice of a pack of wolves or swarms of too many tiny bats to hurt, and, yes, can take over your mind with a glance.
One who has also trained wizardry to the utter limit of what mortal skill can muster at zero risk has all that, plus twenty skeleton bodyguards. And they have had time to prepare and cast all the enhancement-spells they require, before the battle began.
There are, of course, differences by city to how it plays out, city by city.