I don't use slaves (yet), but the V-Rack will hold ANY virtual instrument audio unit and keeps it loaded whenever you switch chunks, so I am pretty sure it works with VE3 as well.
I'm working with V-Racks for over a year now, and it's very reliable. I've lots of sequences for every film (at least one for each cue, and these in lots of different versions), and I can instantly switch between the sequences. I haven't noticed any negative effect on CPU or latency. The only downside is that instruments in a V-Rack are not automatable. If you want to automate, say, the volume, then you have to route the output of the V-Rack VI into an aux track in the sequence. (But you can automate the MIDI volume, of course.)
For me, V-Racks are the killer feature of DP ... read the manual (p. 659) and try it out!
Cheers, Pitt