Without knowing more about your turbine, I would guess the v-belts are a large part of the problem. A synchronous belt would be more efficient and turn a lot easier, but they need more precise tension and parallel shafts. I think your best options are probably roller chain or single stage synchronous belt.
So roller chain it is!
@TDC, you were absolutely right! After further investigations, I did noticed that the v-belt was slipping hard on load. I've tried to find out an optimal belt tension with no success. And there was a lot of "rubber dust" around the pulley - it wouldn't last longer anyway. And, of course, a lot of friction/slipping noises.
Now I've changed every belt/pulley with a single 10B roller chain (2:1 transmission ratio) and everything is ok (including noise). Of course, I had to step up the voltage using a boost converter (at least I finally have something to convert!).
I wrote some kind of MPPT algorithm for the boost converter, by reading the rotors RPM (btw, I placed the Hall sensor along one rotor disk so it's activated by every magnet passing by, for increased accuracy), the generator ouput voltage and the battery charging current.
This way, I gave my VAWT some time to spin up before extracting some useful power and I keep it at resonable speed during normal wind condition. We had some strong gales lately and there was no problem at all (the generator is oversized).
Many thanks to you too, @Flux! You have also suggested roller chains on another thread and now I can confirm: that's the way to go. Sorry for I giving you wrong informations on previous posts - I just didn't noticed at the time that the belt was actually slipping.