those plants look great.
I picked up some so called "thai purple" weeks ago, since already growing some ornamentals, but figured they just had the wrong name for'em (but still wanted to grow them for fun & see what grows from them)
they do kinda look like your Vietnamese purple, but the ones I'm growing have purple leaves & a couple pods are turning kinda reddish/pinkish, I still think mine have a wrong name & I havent looked up Vietnamese purple yet.