I've never actually made a banana cream pie, but I'm pretty sure Jell-o boxed puddings are vegan if you make them with non-dairy milk and come in banana flavors. Could be a starting place, maybe?
Jell-O cooked puddings work well with non-dairy milk. I haven't had much luck with the instant puddings though. I have read that there is something in the instant pudding which reacts to dairy protein and causes the pudding to thicken. It, unfortunately, doesn't work very well with non-dairy milk. Not for me, anyway.