Though I'm not sure what the Vietnamese of the period looked like, I've been thinking of building a force from the Burmese Toungoo Empire as opponents for my Ming. There were border conflicts between the two for much of the latter 16th century, and a war in 1583 - some of the Ming commanders went on to serve against the Japanese in the Imjin war, so they'd be perfect opponents if you already use your Ming to fight the Japanese.
From the small amount of research that I've done, I think much of the new-ish range of Burmese from Steve Barber would work if you leave out the ones with firearms, or convert them to look like earlier matchlocks.
Hopefully an option to consider!