I was actually thinking of doing some thing similar. Using the Musorians as rebels in china or as VPA. How well do the Musorians scale up to the empress moderns?
They are smaller but then it was oft remarked that Chinese volunteers serving with the NVA were generally taller and more robust. If it bothers you creative basing should overcome any problems. It doesn't bother me particularly but your mileage may vary.
Last year I found some interesting photos of Vietnamese defences on the Spratlys. The larger islands have some interesting kit, dug in ZU-23 AA and even the turret of a PT-76 being used as a fortification.
If you are worried about your VPA not being equipped with the latest uniforms and kit, just remember that when China invaded in 1979 most of the Vietnamese opposition was, at least initially, a collection of territorial, second line and militia units, the regulars mostly being in Cambodia at the time.
I did suggest that Empress might look at Vietnamese as credible opponents for their PLA but the suggestion was pooh-poohed. Apparently armies that will, in all likelihood, never face off are preferable to those with an established history of conflict. Allegedly they won't sell and for all I know they are probably right.