VoN 5 is currently impossible to solo. It used to be able to be duo'd (one character on each side, both with a 25 str, 20 wis, and some twitch skills can hit the 2 levers/1 rune). Nowadays, it's restricted to a minimum of three people, due to the furnace launcher (2 people downstairs to hit 2 levers each, one upstairs to receive the blast).
VoN 6 has been soloed many times. I am unsure if can still be soloed, though my gut says yes.