I run VectorWorks on both a dual G5 and a G4ibook (1GHz). I find that the ibook runs VectorWorks really well, all except big renderings. The extra screen the wv_vectorworker talks about would be great.
For all day work the big machine with 2 monitors is tops, for the ability to work anywhere in the office the ibook is tops. You have to figure out which one you want. Do do this ask yourself this:
is mobility more important than power?
If the answer is YES, then get a powerbook, if the answer is NO, then get a G5 machine