For donkey's years I have fitted footrest rubbers, kickstart rubbers etc with a bit of petrol. I know that probably breaks all sorts of regulations these days but you have to be practical.
A wipe of petrol over the peg, and bang the footrest rubber on with a soft hammer. It is very slippery but only for a very short time, obviously. It doesn't seem to do any harm to the parts. Might not washing up liquid stay slippery and let the rubber come off again? I have only ever removed footrest rubbers when they are worn, usually by cutting them off.
That does not help the issue of the splitting rubber though - is it from a duff batch? I don't know of problems with Vincent items but I have had trouble with some poor quality Velo fork gaiters - very annoying to have to pull the forks to bits again within a short time.
Good luck, Pete