It would be useful if the Xvnc plugin could block until the Xvnc server has finished starting-up, rather than passing control to the next plugin immediately.

This would be useful for applications that are started at the beginning of the build, or by other plugins. To give one example, see the Android Emulator plugin (and associated JENKINS-6912).

Possibly we could keep polling TCP port (5900 + $DISPLAY) until it becomes bound, and block until then (or time out)?

I'm not sure whether there can be a universal solution for all VNC servers (e.g. do they all use TCP?), I but thought I'd put it in JIRA and see if there's any opinion.