I have been on the Thai side of the river at Vientienne, at the time I was there, crossing the river was not possible, for me at least. But, I had a grand afternoon visiting with a villager next to the river. The area isn't so rich photographically as I recall. It was a nice bicycle trip from Udorn though.

Chiang Mai on the other hand, and on up into Myanmar would be a lovely trip. I've traveled between Udorn and Chiang Mai by truck, didn't get too much time around Chiang Mai, but it would be easy pickings, so to speak, photographically around there.
If you have the time, taking a train up from Bangkok to Chiang Mai (assuming there is one) would be quite a ride.