First for a Sorcere you need to pick a Bloodline.

Secondly Pathfinder uses CR to determin what appropriate level something i

So Well use an Example a Human level 6 sorcer is CR 5 (level -1)

A Human Vampire Sorcer level 6 would be CR 7 ( the cr 5 +2)

This tell us a level 6 human vampire sorcer starting is appropriate if all the other Pc's are level 8.

Starting off hed be level 8 Xp and have two Dud levels.

I dont know the Math behind it but as he gains levels, the diferende between him and the other party members will lessen. But it will look something like this (and please keep in mind this is by example the actual setup may be wrong i just havent checke dmy book

Vampire Party
Sorcerer 6/vamp 2 Other Pc's 8
Sorcerer 7/vamp 2 Other Pc's 9
Sorcerer 8/vamp 1 Other Pc's 10
Sorcerer 10/vamp 1 Other Pc's 11

Eventually removing the difrences entirely. i cannot remember which page number its on where it explains this.

The over all idea works pretty good but it works better for low CR creautures better than others or something like a Centaur.