Two flights a week are made in each direction between Phongsali and Vientiane. 
The one from Phongsali to Vientiane flies on Tuesdays and Saturdays at 11:40. Tickets can be booked in Phongsali at the Viphaphone Hotel. Vientiane to Phongsali flights leave at 09:30 on the same days and tickets can be bought at Lao Airlines or any travel agent. 
The flights take an hour and 40 minutes and cost around 990,000 kip one-way. The airport is by the town of Boun Neua 41 kilometres away. Numerous buses pass through Boun Neua enroute to other destinations and arriving at the Phongsali bus station at 07:45 to catch one of these will be ample time for the journey. Expect to pay 15,000 kip for the bus.

There have been recent reports that flights are no longer going to Phongsali with the situation seemingly changing on a regular basis. Agents in Vientiane have the latest information on this.