I can't see why not... you might pay a premium though...
I've just 'turned up and booked' throughout Thailand and Malaysia with no problems and know people who have done it in Cambodia, so I can't see why it would be any different in Vietnam...
Make sure there are no local festivals/public holidays around the time and place you want to go, or you could be fighting fellow travellers and city locals for rooms...
Look online too... even the smaller more basic Island resorts offer prebooking via their web sites...
BTW... I'm jealous ;-)