I also dont' use leather and used the Vetta nylon straps for over a year (some time back on Eggbeaters) with really no problems. I don't tighten or loosen my straps with each stop and start so I have them set fairly tight and could kick into them. Eventually, the nylon gets torn up by the teeth of the buckle and that made the strap loosen from time to time. I think I eventually snapped the buckle at some point. But those straps seem the thickest of nylon straps I have seen.

I am now using two different generic/****ty brands of straps, two straps to a pedal, Xed (vettas are too thick to get two through the opening on the MKS steel clips). It seems really nice, probably similar to running two straps straight across without the problem of the front strap slipping foward. The only occassional problem is the straps getting twisted but that has rarely happened.

Lorica, the same matieral that Sidi uses for most of their shoes and Fizik uses on some of their saddles (although they call it Microtex) would probalby make an awesome vegan double strap. The material isnt' that available in the US but Vegan Erotica used to sell it by the yard. The problem would be finding the buckles and then pressing them in. Word on the streets of NYC is that Trackstar and RELoad were once discussing making a vegan double strap.