Hi there,
I did take the bus to Jinghong in May. There seem to be two options daily (apart from Sunday). This info is from the wall of the Phetarloun Bus Station timetable:
Bokeo - Jinghong (VIP): 8:30 AM, Mon-Sat, 160000 Kip, 160 Yuan
Bokeo - Kunming (VIP): 10:00 AM, Daily, 420000 Kip, 320 Yuan
The Kunming bus also provides a connection to Jinghong. In theory they transfer you to another bus in Mohan, in my case the connecting bus never arrived and those of us (3 Chinese and me) going to Jinghong were asked to get off around 10 PM, and a van took us down to Jinghong from the Kunming highway.
So, I actually took the Kunming sleeper bus, as my tuktuk driver took me to the office of the bus company rather than the bus terminal (even though I asked for "kiu lot chiin"). Thai and Lao-speaking staff there said that there are no direct buses to Jinghong (that is THEIR company doesn't run direct services), and sold me the transfer ticket. I got the ticket the previous afternoon, after arriving from Thailand. Only the following day I realised that there was another option. Travel agents in Huayxai seem to be selling tickets for both services, but may charge a commission. On the other hand, tuk-tuks to the new fancy bus station (some 6-8 km away, close to the new bridge) are not cheap either...
I was happy with the service I took. The bus was half-full until the Chinese border. But had I known, I would have taken the earlier direct service.
Safe travels,
KJ