Okay, I enjoy this civ a lot! It has many interesting themes & bonuses.
Free units on DoW: extremely powerful early game. You can DoW for "free" in the ancient era on every civ you meet without diplo or security concerns and get a large army. It pairs especially well with wide play, to deal with barbarians and ensure appropriate defense while aggressively settling. It loses effectiveness later on because infantry units just aren't that good and war become increasingly costly, which is fine. I wish it spawned a slinger though -- I think the Pineappledan's Tweaks requirement date from when slingers weren't the default archer unit?
Wet Foot: I was wary initially, the promotion seemed too weak to give only to UUs and civilians. But the UI is buildable on *so many tiles* that additional movement is *very* welcome. On UUs it also emphasizes the defensive nature of Viet Nam alongside 20% CS against civ with more pop. I like it a lot.
UUs and UB: I don't have much to say. They're all interesting and of an appropriate strength level imo. I like having archers with cover in particular. I'm an archery fanboy, it's my most built unit line, and I'm decent enough at positioning that other archers are my main threat; cover helps with this.
UI: If VP had a competitive scene (like lekmod), this UI would get Viet Nam banned 100% of the time. Look at these tiles:
The fact that it doesn't remove features means it's especially busted on jungles (+1f from feature, +1p from UA) and forests (+1p from features). My jungle cities grew so fast that they had happiness problems throughout the classical and medieval eras despite very good yields and a large balance of national happiness. I also had the most income and culture all game.
It's a difficult UI to fix because it does so many things. For example, yields in jungles with multiple adjacencies are insane but on a plain or grassland coastal tile you get +1f/c/gap, which is ok early game but doesn't scale.
Some suggestions
Terrains: I'd consider limiting to freshwater or even river only.
Feature removal: should stay in; good identity.
Yields: remove culture from city connections (keep production, it's the weakpoint of this biome). Turn gold&GAP per adjacency to per 2 adjacencies.