Comment # 2 on bug 956537 from Michal Srb

As first choice vncserver tries to start desktop based on WINDOWMANAGER
environmental variable which is normally set if you are inside a graphical
session. Second choice is the twm, which can be changed in configuration.

I don't think it is good idea to make xorg-x11-Xvnc depend on twm, but the
default configuration could be more dynamic and try something else if twm is
not available.