That's how fake transparency works. RabidVWM doesn't do the "punch out" transparency because then you couldn't click on the transparent bits. (and I was too lazy to make it work :)

In some cases it fills the background with pink before drawing everything, I could have an option to use a different color. It might be a while before it's released tho, I have a few other changes that are half done right now.

OTOH, depending on the exact effect you're going for, monochrome bitmaps should use the back and border colors instead of black and white. I don't remember if I've actually tested that, but it might do what you want.