Reporting back re: my attempts to recreate the beverage called Vietnamese Salty Plum Soda at the Vietnamese restaurant. Using YT and blog research, a trip to the Asian grocery store, and rigorous experimentation on a series of hot afternoons, I've got something refreshing that works for me. But I won't be drinking this every day because Wow So Much Sugar! Not quite as much as a can of Coke, but not far off.
Anyway, for those who asked for a follow-up, here's how:
One or two pickled plums, muddled a bit in the bottom of a glass with a couple of tablespoons (yikes) of sugar. Add a few ice cubes, then tonic water to the top. Stir. Sip. Yum.
If you try it, I hope you like it!