Can I apply for a 90-day Thai visa while in Vietnam, and what do I need to bring?

You go to apply for the Non Immigrant B visa, which gives you 90 days of stay when you enter Thailand? Or another tourist visa?

Here's the information from the thai consulate in Hanoi

-Tourist-Visa.html

Here's the information from the thai consulate in Ho Chi Minh City

-Tourist-Visa.html

As a rule you get visas for thailand at the thai consulates in other countries,

In Vietnam there are two. One in Hanoi, one in Ho Chi Minh City.

You could apply for a single entry tourist visa (that will get you stamped in for 60 days when you arrive here) at either of them.

You need to provide proof of 20K baht (or the equivalent) in a bank account, proof of onward travel in 60 - 90 days and proof of a hotel booking.

Remember they ONLY accept US dollars at the thai consulates in Vietnam for the visa fees