You could record one, create another track for the VST (Or keyboard.. whichever you didn't record), and copy the track into it.. Dont know if recording both sounds simultaneously is possible.. could be wrong
It's possible, especially if your keyboard has MIDI I/O that can record a MIDI track while recording the audio from it. I'm not familiar enough with MIDI to tell you how to make it work though. It would seem that if you recorded the MIDI part onto a track, copied it, then ran the two MIDI tracks through each source it would be pretty easy.
Definitely possible. I assume when you say the bass sound from your keyboard you're talking about a bass preset on the keyboard, such that when you play the keys the sound is put out through the speakers/line out.
Basically all you have to do is:
-Record from the line out on your keyboard to the line in on your pc/interface.
-At the same time run a midi cable into your pc/interface and use it to trigger a vst synth.
So basically you have an audio track with the recording from your keyboard as well as a separate track for the synth sound.
The specifics depend on your interface, sequencer, synths and keyboard but it's all pretty intuitive.