First, make sure your stock footage doesn't already have an alpha channel. If you know that it doesn't, you should try a simple luminance key. since Vegas Home Studio doesn't have a luminance key option, I use my chromakey FX and select the white background as the key color. You'll have to play the the other parameters to get the keyed effect you want. But for a down and dirty effect, this should work. (Although I must admit that I don't know the video EFX's you have available, I just can't imagine you don't have the capacity to key something.)