This just came to me....before a person transforms into a werewolf for his first time, could he be bitten and changed into a vampire?

According to the books (BD in particular), it states that vampire venom is poison to the wolf pack. That's how they knew Renessmee wasn't venomous, being that she would bite Jacob.

I think even before that they became a wolf, the venom would kill them, it's in their blood (descendants), so I would think that it would still kill them.

nope, sorry. i have no idea. good question though.

Yes they can, because before they become a werewolf they are actually human like everyone else so they can be turned into a vampire, but they can't be a vampire if they are a werewolf.
Hope i answered your question.:)

Im not really sure... probably because their technically not a werewolf though they do have the gene! But nothing has happened to them so their fore there an ordinary human and can be transformed into a vamp!

I think that he probably still could be turned in to a vamp. i mean, hes still human until he first phases.