How do you define 'protectorate'? In particular, i think a good case could be made for this being otl.
This exactly. That's exactly what the North Vietnamese and their political supporters in the South said we were doing. The way to make it distinct from OTL would be if the USA were to fly our flag over it in the sense we did/do over the Philippines or Puerto Rico or to pick a slightly more ambiguous case, the Panama Canal Zone. But what could possibly motivate the US to make such a claim? It would be a challenge indeed to imagine the wacky sequence of events that would lead to the US trying to hold South Vietnam as more or less integral US territory.
In an indirect way I'd go farther than I did above and say that not only did South Vietnam look like a US protectorate to our foes, it objectively was nothing more or less than that. Without the USA stepping in when France pulled out, South Vietnam would simply never have existed at all; Ho Chi Minh's followers were strong enough to simply take over if some Western Great Power or other didn't dispatch resources and agents to stop them. For a time the US was able to hold them off without sending in a lot of actual American forces, via maintaining and advising locals who, in the view of the majority of all Vietnamese, were compromised by their cooperation with the former French regime, which was at best clearly a protectorate, in the brief period after being a colony outright. No South Vietnamese government was able to survive without US support from the get-go; there was no native alternate to Ho that had anything approaching the popularity the Viet Minh did. That doesn't mean that all Vietnamese wanted the Viet Minh; quite a lot didn't, but they had nothing to rally around that could unite them, and in decades of trying the US failed to help them find something. In the interim, nothing but US backing explained why the Saigon government existed in the first place, nor could it last without US--"protection."
How
else would you define a protectorate?