Just my general impression.
For me, people from Myanmar, Cambodia and Malaysia are clearly different from the Thai, but I see little difference between rural Thai in Isaan and the Lao people.
It is not so easy for a foreigner, not experienced in that area, to see a difference who is from one and who is from the other side of the river.
I am not experienced, despite I travelled around that area several times for vacation and I find these people from Laos quite similar to the people from Isaan, even their language is very similar.