One for you networking types

This is probably a silly question, I think I already know the answer, but I'm not 100% sure. If I have a network interface with a low MTU (576 in this case), I can't then create a VPN connection via that interface with an MTU of 1492 and expect everything to work, can I? Surely that's just crazy talk? Just wondering because this appears to be what Ubuntu is attempting to sell me as an automagically configured VPN connection. I can ping stuff, but nothing major is working. I'm guessing ping packets are just too small to be affected by those MTU values. Additionally, any reason why I can't set my interface MTU to as close to 1500 as is reasonable and keep my VPN MTU where it is? I don't know why Ubuntu went for 576, that's all. I've never had an MTU that low before so I wondered if it might have done some form of autodetection (or maybe it's just playing silly buggers). Answers on a postcard to the usual womble.