We've found a few vibrators that work well between bodies during sex.
The Swan is a good one to try because of its length -- it's easy to hang onto it but not get in the way. It also balances well between bodies.
Pele is another good choice -- her "boomerang" shape gives her just the right angle to balance on a woman's leg to hit just the right spots.
If neither of those seems like it would work for you, keep in mind that any vibrator can be held in place by one or both partners during intercourse. Our customers have reported that longer, shaft-style vibrators work better than the egg-shaped vibes.
If you have a male partner, or if you like to have sex with a partner using a dildo or fingers, a vibrating cock ring like Binky, No Strings Attached, Cock Ring Vibe, or Utopia 1 or 2 might be a good choice. They stretch around a penis, dildo or fingers and the vibrator sits on top, to provide clitoral stimulation during penetration. If these are worn on a penis, they have the added benefit of holding some of the blood in the penis during an erection and making it slightly harder and giving the man more sensation.
You could also try a wearable vibrator for her, like the Leather Butterfly. This style of vibrator is designed to be worn like underwear, but with a vibrator sitting on top of the woman's clitoris and plenty of room underneath for penetration. This is a great alternative for people who don't want to hold onto anything during intercourse.