My wife explained that if you can see panty lines, then it is the same as seeing a woman's underwear. It seems there are two types of VPL: those where the undergarment itself is visible, and those where the position of the undergarment's edges can be ascertained through other clothing. Quite different, IMHO.
Wow! It has its own acronym. I have been living a sheltered life.
Also, learned a new word from my wife -- frumpy. Yes, it is a real word.