Have you tried installing in this order:
1) Eclipse
2) Run eclipse, close it
2.5) Install EMF
3) Install VE
4) Install UML
5) Run eclipse and say yes to the new plugins, install them and close it
6) Install MyEclipse?
The UML2, VE and MyEclipse are all very heavy weight plugins, i would try installing them (running eclipse) 1 at a time. I have the VE plugin installed along side my installation and don’t currenlty have any problems, but I admit I haven’t tried install the UML2 plugin.
However if I’m not mistaken the VE plugin did need the EMF (or GEF) plugins…