I just installed beta 4 (I know, I'm a version behind). I tested NVIZ using the GRASS 5.7 Spearfish data set. Mac OSX 10.3.
I should note that this dataset still contains sites lists and these need to be converted to vector points to display. I guess it's good practice in transforming older GRASS files into 5.7 
Anyway, NVIZ does work with vectors, sort of, as follows:
Surfaces display fine (tested with elevation.dem)
Vector lines display fine when called by the 'vectors' panel (tested with streams)
Vector areas do not display at all when called by the 'vectors' panel (tested with fields)
Vector points crash NVIZ when called by the 'vectors' panel (tested with converted archsites of course)
Vector points display fine when called by the 'sites' panel (test with converted archsites)
Vector lines and polygons crash NVIZ when called by the 'sites' panel (tested with streams and fields)
Additionally, the 'place scale object' and 'draw ruler' buttons still do nothing. The place north arrow works most of the time and is quite nice.
Other things seem to work fine. Using the lighting direction tool generates an error but seems to work anyway.
"malloc[2742]: Deallocation of a pointer not malloced: <various memory addresses>; this could be a double free(), or free() called with the middle of an allocated block; try setting the environment variable MallocHelp to see tools to help debug."
As a suggestion, the vector panel should probably be renamed 'lines and areas' (assuming areas can be displayed), and the sites panel renamed 'points' to fit in with the new GRASS vector model.
Hope this helps
Michael
______________________________
Michael Barton, Professor & Curator
School of Human Origins, Cultures, & Societies
Arizona State University
Tempe, AZ 85287-2402
USA
voice: 480-965-6262; fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton