Still I didn't get if such drawback is due to Windows evolution in handling soundcards (XP ... , or VLC own development... or a combination of both... but I clearly recall that I could easily switch to mono mode (mixed left+right) by going through the menu Audio > Audio device, and from there toggle between Stereo and Mono (the only two choices). Now, the same menu behaves differently, listing devices by their names.
The workaround of temporarily modifying system's audio settings (back and forth), relaying on some hardware capabilities (which, unfortunately, are not always available), is becoming quite annoying.
Alternatively, please consider adding to the other menu, Audio > Stereo mode, an additional "Mono (Left + Right)" option, possibly linked to a new specific audio filter (to be included).