Being a bass player I still often grab VSTs during conceptual work or song writing e.g. when the key is not yet fixed. Quite often I'm surprised how well it does in that respect (NI Rickenbacker most definitely, other Kontakt bass libs are also OK).
You need to play what a bass player would play, though, not guitar stuff ... especially, in terms of timing and also how many notes you allow ;-) I don't mean to be patronizing by any means but IMHO that's more often the problem than the bass sound. Quantized straight eights will sound like s*** even using the most realistic bass lib. Never ever quantize a bass, that's a cardinal sin. Even with a keyboard, play until you are in the pocket and fix individual notes at most.