The argument being put forward is that the semi sleeper buses to Mumbai in fact take same time/little longer whereas the VB might be a bit more comfortable so people might prefer VB. End to end even I am not sure if passengers will be there. You may get passengers in intermediate section ( example MAQ-RN or RN CSMT). From MAO onwards till CSMT, passengers might still go with Mandovi express which has better food, sleeper options if time is not an issue.