I am wondering what to do when I install a new vanity in my bathroom and the sink drain and the p trap are not in line. Does anyone have an idea?

Move the vanity ?
Have you looked at some of the flexable pipe that work with P traps that allow you fit. Also many of the plastic pipe allow you to adjust angles and the such. How much off are you talking about 2 inches or 2 feet.

It's amazing what a 22.5 degree elbow can do. If it's a very small amount then rubber Ferco couplings can be used. Some drains also are corrogated a bit to allow for alignment.

As Fr. Chuck said 1/4" or 10' and in what direction.

Yes, I had to do one about a foot off on my last flip ( wife says last time I buy a vanity on my own again) They actually sold some product at home depo just for this. ** now being a man I would not buy it but decided I could design my own with various angles and fittings..

How out of line is it. If you talking 2 to 4 inches you can still work that with a regular trap. They do make flex pipes you can use though not a big fan of that, also move this question to plumming where speedball can see it

Speedball, at the plumbing page, offers great - and accurate - advice for plumbing problems.