Upvote:1
I've just come back from Vang Vieng and you can indeed catch a bus which goes all the way to Chang Mai as well as many other places. You can buy a ticket in advance from the departing bus station which is located in the southern end of town near the old airport runway.
Upvote:2
There are no through buses between Vang Vieng and Chiang Mai, basically you have the two choices that you listed: