USA passport gets 60 days free visa exemption entry into Thailand..

The day you enter is day 1 of the 60 days so keep that in mind.

BUT ...

You will need a visa before you travel to Vietnam..

THERE IS NO VISA ON ARRIVAL IN VIETNAM for US passport..

Apply online for Evisa to Vietnam

That's the official immigration policy but in multiple entries over many years I have never once been asked about funds or anything else.

I doubt very much Thailand is likely to change from the current 60 days visa exemption entry.

They are trying to encourage more tourists into the country so I wouldn't be concerned about that.

@Brandon ***********

Your information is not correct..

Visa run agencies are using that border as I know 2 people who did a visa run today and all went well.

And it's not their first time to that border either

Seen hotels check that many times in the past...nothing unusual

It seems to depend on the local policies at each immigration office..

But many seem to recommend you go around 7 days before your initial 60 days expires

No, don't worry about it..

Postponed

My understanding is anyone who enters Thailand on a Visa exemption entry needs to be able to show they have available at least 10,000 Thailand Baht for individuals or 20,000 Thailand Baht for a family..

That's is an immigration rule..

But in reality it's very rarely if ever checked or any questions asked about funds however it's still a possibility of course so you should be aware of that..

It's quite likely your airline may ask you for proof of onwards travel out of Thailand within your 60 visa exemption entry...

You are expected of course as a tourist to be a temporary visitor and exit Thailand within the rules so sometimes the airlines will ask.

You need to check your visa requirements for Vietnam..

Not sure on French passport but many passport countries must have a visa before travel.