You need a product like Sonar or cubase. When you have a midi file it has to talk to something to send the messages to then the reaction would be for that product[keyboard audio outputs to soundcard inputs,vegas track record 1-2 left/right]to play the audio to be recorded. You can't just import midi...its not audio...only a language to tell a device when to play and what.

In cubase I would get a midi file from a friend on a floppy and in cubase open that file to be used. I would sync his korg triton to my daw via midi and press record on cubase. The info would then lock up both the keyboard and my daw and would then start playing the midi file. You could change the sound patches on the keyboard so that the bass line would have a different tone/character/sound bank/etc, the same sixteenth note bass line mind you.

The way you described you would need DXi's and that is only available through Sonar right now. You could sync midi time code to a keyboard yes. But all the editing must be done at the keyboard. Vegas would be the Master...DAW/Vegas the slave. Line up the start points and press record. After you've finished recording from the keyboard mix to a stereo 16bit 44.1 wave and your done.If I made a mistake someone please correct me or tell me of a easier way of getting it done. Hope that helps. Later.