Easiest option. Cross the border without a visa, as noted above. But..

If you are going to be in Vientienne anyway, you MAY choose to get a tourist visa (about $30, a pickup and drop off trip) at the Thai Embassy in Laos. This will get you 60 days that can be extended for 30 days.

The only reason I mention the visa option is because of the number of people I meet that have visa problems when plans change. Sometimes people decide to stay longer. Sometimes there are medical emergencies that people did not plan. But, in every case, having gotten a tourist visa would have all but solved these issues.

Since the cost is almost negligent, and you will already be in Vientienne, it's really not a bad option. The 15 day entry stamp, that cannot be extended, has limitations.

Just a suggestion.