The first one: Beryl & VNC
VNC does not currently support accelerated GL rendering.
Some workarounds are though possible:
- use x11vnc with the -noxdamage switch
- take a look at virtualgl (at least since 2.0beta there is a MacOSX Intel client, nevertheless it is not part of the FreeBSD ports tree…)

The second one concerns multi-head systems:
Xinerama disables the RandR extension so Xinerama is unable to work with Beryl since it requires that extension.
Beryl (Emerald ?) is unable to manage the second head presents in my system, so I can start Xorg with 2 screens but the second one will be unusable.
As of today, you have got two options:
- at lest, dual-heading seems to work with nvidia TwinView (which offers better OpenGL performances)
- use one window manager per head (ie: run two instances of emerald)