I'm not sure the domestic lounge situation on Delta really matters to me, the last time a domestic lounge saved me any grief rather than adding, it was USO lounges. USO lounges are actually much nicer than what the airlines offer, I still appreciate the people who volunteered back then.
It occurs to me that I have seen a number of dedicated Vietnam Airlines lounges in the international airports. I think that, depending on your specific route, Vietnam Airlines can probably save you a lot of headache over other carriers, but it's a question of the caliber of the tourism spot being served on the route.
Local people willing to pay a slight premium, versus international tourists booking award travel from partner airlines at no cost.
I'd have to look at how the points earn is accelerated, and how award flights from Skyteam partners contribute to qualifying flights and segments. The rail system in Vietnam is a much better experience traveling than elsewhere in Southeast Asia, but it is antiquated and low speed, there are a lot of reasons to fly around instead during a visit - most travelers will stay longer on single-entry visas because of a lack of visa-free entry, no last minute hastily planned border crossings.