By default in OS X, Vassal creates a directory called VASSAL, for
storing preferences and such, in my main user directory:
Macintosh HD:Users:Tim:VASSAL
Is there a way to change this location? I’d like to put it on Dropbox to
share it across several machines.
Not presently, no.
It’s also likely to become infeasible to have that directory be located
in something like Dropbox in 3.2, as we’ll be storing the on-disk tile
cache there, which will potentially be quite large.