You need to purchase a visa on arrival to enter Laos. You also must book an appointment with the Vientiane Thai embassy in advance. If you just show up without an appointment you will be sent away.
A single-entry visa is only good for one entry. It cannot be used twice. You can get a re-entry permit before you go to Laos which will keep the visa "alive" while you visit Laos, costs 1000 baht for a single re-entry. Or, you can apply for a new visa at a Thai embassy or consulate in Laos.
Get a new card from the rear of the checkpoint area before you get in queue to stamp out. If you wait for the officer he will give you a new one and send you to the rear of the queue.