Also even with the Air Asia late evening arrival you can make it to the border in time.
It's actually easier and cheaper to get the minivan from the airport to the border than to go into Udon. Because from Udon you have to then take another mini van or bus.
It is well worth the (small) money to pay an agent to get your visa. You don't have to wait in any lines and they will pick up and drop off at your hotel.
Then you can spend your time enjoying the sights. And yes, there are actually some things to see in little Old Vientiane.
Have you looked at the pay? Most government schools pay 25-30,000 Baht a month. Less than $1,000 USD. Some private schools pay more, some around 60,000 baht, especially in Phuket.