Based on your question I'm assuming you're testing on a wireless network. So. Here's my experience with the same problem. I've successful used the MiTM attack from vmware bt5 to vmware windows xp and 7 and physical win7 via ethernet ports, but when I tried the same attack via wifi physical(but vmware's eth0 bridge), it failed miserably. So... I troubleshot. Long story short, if you used an external card, it should work find. I have an ALFA AWUSO36H, and I, the forums, and the rest of the general web, recommend it highly. It only costs like 25-30 bucks. Not sure why it fails to do it the other way, I assume it's a problem with the..... Actually I have no idea.... But if you do find out why, please let me know.