a METV can only be applied for in one's home country, most embassies or consulates in neighbouring countries don't offer it to non-residents. Given this and the fact that now all neigbouring countries stopped issueing VOA or visa-exempt entries, there remains only one way - that's the road back to the home country
as US passport holder, you can still do a border bounce flight to Vietnam. There is a list of countries whose citizens cannot do a visa-free entry to Vietnam any more, and I think, the USA are not on that list. However, boarding a flight to Thailand in Vietnam could pose to be impossible without presenting a medical clearance and a travel insurance covering a covid-19 infection up to the amount of 100,000 USD. Please check on the US embassy Vietnam website for instruction and guidance
I have been crossing this border a few times, it is used by many foreigners, too. And actually an easy going in-and-out border. The main road from Ubon Ratchathani to Paxxe, pretty busy everyday
the IATA list from today, 9:15 am, does not mention any requirements for US citizens who have been to Cambodia, to present anything upon entering Thailand
Chong Chom is a friendly border point, but known they keep asking people for the proof of accomodation for the first few days in Thailand and 20,000 THB or equivalent in cash, and an onward ticket. At least three of my aquaintances happened to be asked at the Chong Chom border when doing a 30 day visa exempt entry. For