Bor Pen Yang — "no problem" in Lao — is Vientiane's longest-running expat bar and still the easiest place to find a crowd at 10pm. Three floors of pool tables, sports TVs, and live music lead up to an open rooftop terrace with one of the best views over the Mekong. The food is forgettable, the cocktails are cheap, and the music gets louder as the night progresses.
It is brash by Vientiane standards but reliably busy. If you want a quiet evening, go elsewhere. If you want to meet people on your first night in town, this is the obvious place. Open until midnight or so on weekdays, a little later on weekends.