There is no required level to become a vampire. However, there are only two ways to do so.

1: get bitten by a certain breed of bloodfiends only found in very specific locations in the level 38-45 maps (bangkorai, reepers march and the rift).They only spawn there at night and only when there is a specific moon. So getting there and staying alive long enough might be harsh.

2: get bitten by a fellow player. To do so you need to be in the same map in front of the vampire shrine. This is not as hard as it seems. You can travel to the fellow player at the wayshrine and he/she can escort you there. The foes are not particular nasty, so it is doable while being severe under level. Keep in mind that a vampire-player can only bite someone once every 7 days.

Although vampirism is wide spread on the pc-version, I imagine it is still rare on the xbox servers. The reason is that only those who got the transfer from pc to xbox have the dissease and it is not even sure if the bloodfiends allready spawned once on the xbox.This means that most vampire-players will charge people to bite them and the price is likely to be higher then on the pc.

So you might want to postpone becoming one till later. I suspect that in a month or 2 it will be more widespread and it will likely be easy to get a bite from a guildie or friend for free.