Practice your vocals this week and next rehearsal, sing the songs yourself (better yet, you and the guitar player swap vocal duties) so your band doesn't need anyone just hanging on (musical wise) and the drummer can't refute the reason to let her go.
Also, if the drummer quits, do an acoustic/low volume duo with the guitar player and you doing the vocals until you work out other arrangements.
You'll improve your musicianship and your marketability as a bass player, bassically, for the rest of your life.
Even if you're not that excellent at vocals, I suggest you just start and keep on working at it until you guys get it down. If you both sing and have 40-50 songs worked out, you'll also be able to attract better drummers.
Btw, I'd bring back your girlfriend since not having her around hasn't created less drama (hey, she's not in the band, anyways...). That way, if you stick around, at least you can be together more and maybe she'll even find your group a gig or two and give you a real reason to let the singer go (can't sing well enough for a gig...). Make some recordings of your rehearsals and it'll be obvious as to where the problems lie.
The other lesson is, even if the band is crappy, you don't have to be. Don't let anyone drag you to a lower lever. Always be prepared, be professional and nail the songs so you are NEVER the weak link in whatever musical situation you're in.
Good luck.