VP$IP = Voluntarily Put Money Into the Pot.

It's all determined pre-flop. Any time you voluntarily put money into the pot, it counts. That includes when you complete the small blind, or call a raise when you're in the big blind. The only time it doesn't count is when you check from the big blind, or you fold.

It's pretty hard to calculate it yourself without pokertracker.

Typical "tight" play has a VP$IP of 15% - 20%.