Save your money and stay away from vBauction (I bought it and it was so buggy it was useless).
A program called PHP Probid is not hard to integrate. As far as a hack is concerned I can't publish Probid code here or vB code there so it is not likely to happen, but it is fairly straightforward to share the user database.
Regards,
Matt
My Friend have php ProBid but the one i just put for the phpBB is more better then phpProbid and it is free to modify it to make it work with vB