I definitely would favor Luang Prabang over Vientiane.

Luang vs. Chiang Rai is a tougher call but I'd give a slight edge to CR. Ten years ago, I would have gone the other way. The tourist flood gets to be too much in Luang.

CR has a (much smaller) night market and a daytime food market, a river offering various excursions, and a temple reflecting Burmese influences. It is a gateway for visiting various hilltribes via trek, elephant, car or boat.

An easy day trip takes you into the heart of the Golden Triangle and, depending on the political situaiton, you can make a quick jog across the Myanmar border to Mae Sae, an interesting trading town. Back in Thailand, you can visit the remote headquarters of the legendary drug lord, Khun Sa.

Good restaurants and accommodations can be found in both destinations the OP is considering.