When I inquired with my now-no-longer-Isuzu dealer about the timing belt, I was quoted under $400 for the job - CHEAP! I would bring it to a dealer as they've no doubt done quite a few Isuzu 3.5L t-belts before, if maybe not in a VX.
Of course, I had the indie shop that did my S/C install do the t-belt at the same time, and paid like $700 for the job. :-(