The question is in the title I love being an evil vampire and all, but it is true that vampire attacks on cities are more frequent when the sun is blotted out ? I don't want to lose all my merchants and quest givers ... I'd rather assassinate the people I want on my own

Easy answer? yes

Yes, vampires, death hounds, and even gargoyles will be more frequent whilst darkness reigns.

Hmm. I blotted out the sun twice just to see the effect and to hear what the NPCs had to say about it. Once in Whiterun, one in Riften. Neither time did I have a vampire attack. I had finished the MQ on the DG side, so maybe that is why nothing happened.

Worked for me and I finished the MQ on the DG side as well. But you have to either blot out the sun while you're not within a city and then travel to a city, or else do as you did before (blotting out the sun while inside a city) but then enter a building and come back out again. You should see an attack.

I blotted out the sun in Solitude, went into the Blue Palace to troll the resident Vampire Mage. Then went out... vamps spawned and attacked NPCs.

I blotted the sun near a whiterun almost immediately did death hounds, gargoyles, and nightlord vampires start attacking. That didn't take too long.

but YES vampire creatures spawn more often in a day with a blotted out sun (Vampires, Death Hounds, AND Gargoyles)

True that. Futhark is correct

You... After many "fubarks" and "fuqarks", you are the first individual to know the obscure meaning behind the "t". Cheers! lol