VISA for SE Asia!Watch
Lucky for you as well Thailand has recently changed the rules so that you can stay for 30 days when crossing into the country by land (before it was 15) so you have nothing to worry about.
Thailand - On arrival (before you could only get on-arrival if you arrived by plane, apparently you can get on-arrival now if you arrive by land as well - double-check it on Google!)
Vietnam - Require visa before arrival however on-arrival is possible but risky. I flew there this summer and there were travel agents online offering 'visas on arrival' which were significantly cheaper than the embassy visas. A lot of people advised against it as a lot of them tended to be scams. However, mine was perfectly fine and I got through with no problems.
General advise: Carry plenty of passport photos! Having visas ready before you fly is always advisable. And use wikitravel but always verify twice the information you find on there or any other travel forum.