Definitely different respawn counters, Oth.

On the Whiterun - Rorikstead road, I get thieves every 3 - 5 days, and vampires at what seems like a week or two. In fact, I'm thinking that Dawnguard may have extended out all the respawns as I'm having a devil of a time finding bandits and thalmor to kill.

There are also different respawn times on types of vampire encounters, random occurs more often than specifically player character targetted. And I've spent a lot of time post DG mq, and haven't had a vamp attack in a town or city since then.

Most devastating attack yet (besides the times I reloaded to save Adrianne from one-shot kills).

A vampire (health 120), vampire mistwalker (health 331), and nightlord vampire (health 1031) just invaded the market place in Riften in the middle of the day. What a mess! The NPCs were so thick I couldn't do anything but toss vampire bane spells into the middle of the melee. Two NPCs died, neither of which I cared about. Indeed, I am glad one of them is gone. Aerin is now an ashpile. Mjoll is quite upset. Nura Snow-Shod was decapitated. Gross! Fortunately, no vendors died.

To quote from the uesp wiki, nightlord vampires "have over 1000 health and are capable of beating just about any other creature or NPC in the game apart from the strongest dragons and a couple of unique NPCs" And this is what Bethesda decided to toss into the middle of cities filled with merchants and quest givers?