Vang Vieng is small enough to walk end to end, so the choice is really noise level: the old airstrip strip, the river bank, or the paddy fields across the bridge.
Main strip and the old airstrip — everything at the door
The flat grid of guesthouses, agencies and bars either side of the disused airstrip. You can book a balloon, rent a scooter and eat within two minutes of your bed, and you will hear the bars until they shut around midnight.
Nam Song riverbank — karst views from bed
The lane along the east bank has the terraces and balconies that face the cliffs, so you get the dawn balloons without leaving your room. A couple of hundred metres of extra walking buys a much quieter night.
Across the toll bridge — rice fields and bungalows
Bungalow places on the west bank sit among paddies and buffalo with the town lights over the water. You need a bicycle or scooter, and the crossing bridges charge a small toll each way.
A note on the train station
Vang Vieng railway station is several kilometres out of town on the far side of Route 13, not walkable with luggage. Ask your guesthouse to arrange the pickup, or agree the shared-van fare before you get in.