I don't know virtual bassist, but:
.) open the midi Editor in Reaper. check the Midi channel that is assigned to the notes (right click on the note). I don't know which channel(s) is (are) interpreted by this VSTi, most likely it's channel 1. (EDIT: found the manual online, It is
channel 1, though, all channels are interpreted - but differently. It might be worth reading: ftp://ftp.steinberg.net/Download/VST...al_Bassist.pdf
.) depending on how low the notes in your file are, i take that most of these bass-vsti's don't go below standard E - everything below just doesn't get interpereted (result: you don't hear anything). Mark all notes (Ctrl+A) in the midi Editor and move them up, try again if you hear now something when you press play. (EDIT: if you play lower than standard E, I found now that the VSTi has a 5string bass model also that goes to low b, so you most likely are limitied to this model when you player lower than standard)
Now, after I found the manual, I saw again something worth checking:You need to set it to "single note" modus, otherwise it does not understand the midi files with notes in it you are sending. There is a "Midi IN" virtual-LED - see if it lights up when you press play.