Sorry - Lonely Planet says this:

Visas: Bureaucratic hassles will be your first problem in getting a visa - expect delays of five days or more. Bangkok is the best place to get one. It's usually best to get your visas through a travel agency. Expense is the other problem; tourist visas valid for a single 30-day stay cost about 40.00 in Bangkok.

Glad to hear that people have better experiences.. but for a 2-3 night sidetrip, I don't know if that would do SGN/HAN justice.. but who knows? We'll see about the logistics of the sidetrippers.