I ran into the following errors when trying to start another Xfce session for VNC while an existing one was already running.
/usr/bin/startxfce4: X server already running on display :1
Unable to create /home/niklaas/.dbus/session-bus
Xlib: extension "RANDR" missing on display ":1".
As a quick workaround, I found that adding the following two lines to
~/.vnc/xstartup allowed Xfce to start successfully.