Configure the Xvnc command line to invoke. Leave it empty to let Hudson figures things out.
Xvnc must be installed on the system
A typical option looks like "/path/to/vncserver :$DISPLAY_NUMBER -geometry 800x600" See the man page for more details. Hudson remembers what display numbers are in use, and when it launches a new vncserver process, it picks the available number and replace $DISPLAY_NUMBER by that number.