Bastoune, I'd suggest a second speaker.
Having a different point of source for the vario audio is better than having the same point of source as the radio. Listening to mono mixes, things are not as easily identifiable within the listening material. From experience with stage monitoring using early wireless headphone systems, you end up with "everything louder than everything else" when using mono. Probably not what you want in a glider.
Late model Schleichers have dual speaker positions near the pilot's shoulders, with speaker wire to both. Having vario on one side and radio on the other is very nice and doesn't require much amplification. Any of these audio devices will use more battery power the louder you turn it up.
If you really don't want to have a second speaker, there are transformers available to drop the speaker output of the vario down to line level.
With a radio having auto mute of the line input, it won't be very helpful to lose the vario signal each time someone pushes the PTT, and the switching the radio off trick won't fix it.
Jim