Generally VidCap works best with a digital signal coming in over a firewire connection. It's not really intended to use anything else. Sometimes you can get lucky and use other capture cards but there's no guarantee. I had some success with a Hauppauge WinTV card, capturing AVI in some of the modes.
If you really want to use some sort of 3rd party capture card then you should capture with whatever software that card comes with. After that you can use the files in Vegas for editing.