I'd like to join the Vampires in Dawnguard's DLC, but I'd rather have Serana bite me instead of Harkon.

Any way to turn down Harkon, but still work for him, and later let Serana bite me?

I've never done his missions, just Dawnguard's missions. When I win, do I keep Volkihar Castle as a home? Also, how can I be sure to unlock the house southeast of Solitude and north of Morthal?

http://www.gamesas.com/forum/178-skyrim-cheats-hints-and-spoilers/

No. You need to accept Harkon's offer in order to be counted amongst the family.

You'll have to accept his offer at first, but you can cure yourself then allow serana to bite you again. and the castle becomes yours ones you finish the questline.

I'm curious about something else. If you join the Vamps, do the random town attacks still happen?

If I feed on someone, do they die or become a Vampire?

Neither. They become hostile if you're detected though.

Does it make me sparkle later on, or is that just a rumor?

Sparkle? no dont think so.

And i think the vampirs attacks stop as soon as you advance in the DG questline. but im not sure.

No. Bethesda didn't make Twilight vampires.

Thank Molag Bal for that.

Serana comments that you're sparkling when you're in the cave with the ancestor moths...

And she DOES slightly resemble Bella Swan...

If you join the Volkihar, there are random attacks on you from Dawnguard members, but they never seem to happen to me as often as the vamp attacks. And they happen more out in the wilderness than inside towns.

Kinda' weird... Why don't you ask him if his daughter can bite your neck instead?