Vietnam Embassy in Vientiane is just north of Patuxay Monument on the road to Pha That Luang (just past a Toyota car dealership on the right, known by tuk-tuks as Patuxay Toyota). The Vietnam Embassy is also shown on map in Lonely Planet guide.
Consulate entrance is around the left-hand side of the Embassy building.
Opening hours: Monday 9:30 to 12:00 and 13:30 to 17:00
Tuesday to Friday: 8:30 to 12:00 and 13:30 to 17:00
There is a one-page, very basic page to fill-out (passport info, job, nationality, address). Don't need to list hotel or any other information. You can choose single or multiple entry (I chose single and don't know details on multiple).
Bring one passport photo that you glue to the page (glue stick available there).
I arrived at 9:30 AM on a Monday and they didn't open until 11 so be prepared for erratic opening hours, but other than that it was fast an easy process. There were about 7 other foreigners applying for visas the day I went. Everyone seemed to be getting the same day service since the price difference is only $5.
Price for Single Entry Visa: Payable in U.S. dollars
USD $65.00 for next-day service
USD $70.00 for same day (apply in morning, pick up in PM)
You can walk, or better ride a bicycle or motorbike to the embassy and combine that with a visit to Patuxay Monument, Wat Phonxai and That Luang. Happy Travels!