Google is unable to find one in Chiang Mai either. Consulates in Chiang mai, Thailand

In Bangkok travel agents often have visa services but I'm not sure if they do it in other cities in Thailand.... not sure I'd risk it even if they do as it would probably involve sending your passports to Bangkok by post.

The Vietnamese visa is a bit more tricky than some ... you need to state the ports of entry and departure for example. More info here. Vietnam Passport and Visa Guide

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.