This is interesting. My parents grew up in the north, so I understand when they speak.

The thing is, even if I'm not very good at it and I've never even been there, I can tell what part of Vietnam someone is from. It seems the southern dialect is very easily distinguished from the northern one.

It makes a difference to the diaspora people because it gives you an idea of whether you're of similar background. The exodus seems to have had different stages and groups (eg people who left with the Americans vs Hoa who left a few years later).