I've tested this several times, using varioud VSTs. I've even tried using a VST (Amplitube) and set to bypass every single processes of the VST plugin and the output I get (after going through the VST) is left and right playing the same thing.
After various attempts, I found out that Live4 mix both left and right channels of audio into left channel and passes only the left channel to the VST effect. After the effect, the VST plugin would spit out a stereo output but with left and right channels producing the same thing.
This phenomenon can be seen clearly with the CM Vocoder plugin.
Note that there hasn't been any problems with Live 3.