If I join the vampires, will I still be able to access Fort Dawnguard (buy stuff from shops, accept quests, ect) or will everyone flip out and try to kill me as soon as I get there?

Are you talking about being a vampire to begin with or joining the Volkihar clan?

Joining the Vampire clan.

Well, you'd be a vampire walking into a fortress full of people who dedicate their lives to hunting and killing vampires. So they'll be about as civil as you'd expect (not even slightly).

^^This. The only thing you will be able to do at Fort Dawnguard if you join the Volkihar is get into a fight with everyone in the fort. That is all.

Surely this is a joke question, isn't it?

Once you join the vampire side of the Dawnguard DLC, you will not be able to go into Fort Dawnguard except to fight them, they will be hostile in sight... it is the same if you join the Dawnguard, after you take a certain someone home, the Vampires will be hostile on sight. It is one of the few choices that really matter and have adverse affect's.