From what I can tell a conversion from R12 to R134 almost always kills the compressor after a year or two (my Astro van compressor lasted two). From what I'm told it's not that the original compressor is not R134A compatible, rather that the oil used with R12 doesn't get along well with the oil used with R134A, and to get 100% of the old oil out is of course next to impossible. What I found in my case was gobs of black "gunk" that had clogged up the orifice tube.
When you buy a new compressor it should be compatible with either type. Just make SURE that you flush the crap out of every part of the system...you won't believe the black gunk that will come out. Also make sure that you replace the dryer/accumulator (even if it's only a year old!) in addition to the orifice tube/expansion valve (not sure which one the vette uses), they usually won't warranty a new compressor unless both are replaced.
I've been quite happy with the Astro's conversion with the new compressor. Although it doesn't work terribly well in stop-'n-go traffic (big surprise there right), it works quite nicely as long as it's moving.
Good luck.
Bill