Read Peter Gannon's 'no brainer' post. you can get the TTS1 soft synth for fee with a cakewalk download. ( you don't need to sue cakewalk - you can use the TTS1 in BIAB). I think the sounds are pretty good - maybe not as good as same specialist dedicated sample sets - but as the sounds are Roland sounds usually ok in a mix.

maybe you can use the TTS1 in Logic pro - i don't know. But if you can't, export the BIAB midi tracks as audio and then you can mix all audio tracks - including your new vocals.

if you are happy with the BIAB mix, maybe you could export the whole backing from BIAB to one audio file.