I can just about do it but quite badly. Either the vocal or bass line takes over
Usually it comes down to practice .. Just keep rehearsing until it becomes apparent
A trick I employed was to disengage the vocal from bass line- disassociate the hands from the mouth !!
Play bass like robot / automatically and once you've got that then all you have to worry about is the vocal because that's more dynamic than the bass..
Not sure if this helps but it is a difficult thing to do..