It looks like stevia, if its very sweet it should be some variation stevia.

Luciano, Thank you. I've found it though. It's called 'Ngo Om' in Vietnamese aka Rice patty herb. It's not sweet, the falvor is more akin to a mild citrusy-cumin, the leaves are much smaller than Stevia. I've potted some of the cuttings up and they've rooted and have begun growing.