The assumption would generally be that if you're running a bass VSTi, then you're recorded a bass track to start with.
I don't know of anything in particular that will convert a guitar line down a few octaves and make it sound like a bass in any convincing fashion.
There are plenty of MIDI bass synths but they tend to sound like bass synths, not bass guitars... put perhaps you can try what IK Obi is suggesting... a bass synth through the free Amplitude Custom Shop thingy which has a bass amp on it... might get you halfway there..
What did you use in your sig tune? Sounds muddy. Some EQing might help it a bit.