They have a lot of pseudo-instrumentals anyway (for instance from Junta "You Enjoy Myself", "Divided Sky", "David Bowie", and "Dinner and A Movie" all might as well be considered instrumentals.)
Phish's vocals are nothing to write home about but singing is nonetheless a pretty important part of the band since they did develop into pretty accomplished songwriters as the years have gone by.
Their 90s-era is far from their vocal peak. Actually they're in better vocal shape these days than they were back then cus you can tell they actually do proper vocal exercises and warm-ups now which they didnt do then. Doesn't really bother me, vocals are always going to be among the most inconsistent aspects of any band live and Phish are some of the whitest dudes ever so the whiteboy vox are pretty standard. I always through Trey kinda sounded like Ben Folds a bit.