that is the dilemma isn't it? Bravo to you for not throwing away all your old kicks and really getting the most out of them.
personally I don't buy canvas shoes because I know how quickly I'll shred through them so I always go for a synth model (or at least something with a rubber toe cap).
Synth is kinda questionable too in regards to environmental impact but don't go thinking for a second that leather is somehow the "natural" alternative as dying leather is probably equally as destructive as producing synth.
I wish brands would experiment more with eco friendly fake leather like Italian microfibers etc. but I doubt any brand out there is really focused that much on the "cause" to do so. I have some amazing micro fibre boots that age just like leather and while they were pricey I've had them for 5+ years and they don't peel like your average PVC/ pleather versions.
I think I-path was probably the best brand we've ever had because they also tried to focus on environmentally sustainable materials like hemp and had a great mix of styles (i.e. not all vulc boat shoes). I'm hoping someone influential like Kenny Anderson can come out with something that ticks all the boxes for us and while I'd like to see another Mike V. shoe, it doesn't look like he'll ever get another shoe sponsor

I think I might have to just bite the bullet on those Ryan Lay etnies because it seems like he's the only current pro that is really pushing the message so to speak...
edit: any of you rocked these?
http://emerica.com/eu/footwear/herman/herman-g6-vulc-6102000097.html?dwvar_6102000097_color=544#q=Vegan&start=3 I've found some pretty cheap but need to know if replacing that (red) insole is a possibility. Sorry, can't do red...