My experience seems to be that the performance of NVIS dipoles (at or
under around 30 feet high for 80 meters) doesn't noticeably change with
orientation. In other words ... and despite what modeling programs say
.... BVIS dipoles are pretty much omnidirectional.

I haven't scientifically tested that but it does seem to me that since
most NVIS radiation goes approximately straight up (definition of NVIS),
it really shouldn't matter much how the antenna is oriented.

Have you all found that to be true, or false?

If I do want near-omnidirectional performance, in the real world (versus
in a software program's modeling), am I better off with an inverted vee
that's up 30 feet in the center, vs a dipole that's 30 feet up along its
full length?

How about an inverted vee that's up 50 feet in the center and 10 feet at
the ends, versus a dipole that's 30 feet up along its full length?