HCMC is a small office, so you can be in and out in minutes. Vientiane requires an appointment, and if you have to cancel that appointment, it can be difficult to reschedule. It's also very heavily populated, and regularly serves 200-300 people (or more) every day.
The only thing I know about the PP embassy is that it takes 3 working days instead of 2 to get your visa. I think this unfortunate fact puts a lot of people off. If you decide to go, I know there are people here who would appreciate hearing about your experience afterwards...
Good update, thanks for posting. Did you provide financials? What specfically did you provide? Have you done this same visa at this same embassy previously? Congrats!