Or, if money is an issue, you might consider flying to Udon Thani and then proceeding by bus. It's not that far away. I'm not sure what the cost differential is between flying into Vientiane vice Udon... Somsai, Rufus, this would seem to be your area of expertise.
Well, as mentioned before Vang Vieng doesn't have an airport -- the nearest is in Vientiane . Also, getting to Vientiane would be 2 flight cuz there's nothing direct from S. Thailand to Laos, you'll have to stopover in Bangkok.
The Bangkok -- Vientiane leg is owned by Thai Airways, and it's not cheap. Their website says it's 6,600 B (about $200 USD).
On the other hand, AirAsia flies from Bangkok to Udon Thani for about $35 USD.
In your situation, I think I'd just fly back to Bangkok from South Thailand (AirAsia does it very cheaply) and then take the train up to Vientiane (a relaxed, scenic ride) then the bus (your only option, really) to Vang Vieng.