When I come to bits I can't do together, I sit down and figure out exactly how the bassline and vocal line intertwine within each beat of the bar, much like playing the melody and bass on a piano I guess. Put the metronome on really slow, and work your way up to speed. Don't be afraid to start it craaaazy slow, I often have to.
I agree with Bass Bunny too. There's a great tip I saw of Yolanda's, where she's playing a syncopated bassline, and just counting the beat out loud over the top, first in crotchets, then quavers. Might sound easy, but it can get pretty difficult. Also try playing the same bassline, and having a regular conversation with someone at the same time without losing the groove. All this stuff helps me!