@Henrik ***
There is no agent at that border. Only thing they have is some "fixer" that will hold your hand in the process and then drive you to Vientiane. But the numbers of people paying for such useless service is very low (only some first timer). Nobody is losing time over not doing this.
Second, if you take a group bus you won't save any time at the border, unless you don't know the process at the border to begin with. You follow the queue like everyone else, you do not skip line. In fact, the reason why the queue can be long is if you arrive at the same time as a few of these buses that go from Udon to Vientiane. If you are unlucky you arrive at the same time or slightly after and can be a lot of people ahead of you. But if you plan ahead and know went these buses usually arrive, you will be fine.
And on top of that, OP is talking about a border bounce, not visa run. Border bounce mean u cross the border, turn around, and come back immediately to Thailand 10 min later, or few hours if you want to shop around. You do not go to Vientiane went you border bounce.