I would say Hanoi is probably the fastest and easiest. I was there at the beginning of the month, Hanoi is a great city to be stuck in while you wait for the visa approval
Nobody can tell you for certain. Maybe immigration will stamp you in no questions asked. Maybe they will ask to see return ticket, hotel booking, cash on hand. Maybe they will stamp you in but tell you to get a visa next time. Maybe they will refuse you entry and make you buy a plane ticket out. It's a crapshoot.