I know you said you think Vox isn't fluent in Korean, but is he largely able to understand spoken Korean? Or is there a limit to how much spoken Korean he can understand also?
Outsourced this answer to my co-conspirator @dallacuna-b, who is Korean themself and, that aside, has more linguistic experience than I do! (Plus it’s nice to delegate when chronic pain is beating my ass LMAO)
Lacuna: This is a good question, but one with a that needs a lot of context and nuance. The answer to this depends on two different variables: Historic Context, and Social Context.
To keep it brief, on the historical side, as established in the canon of the show, it's been around, possibly more than 70 years since Vox died and went to hell. That's a lot of time for language to change. Take for example, just English. Even though we can understand the bulk of English from the 40s and 50s, there are some things we would wholly misunderstand without context. Take, for instance, these famous panels from Batman #66 The Joker's Comedy of Errors! (1940):
These mean something very different to us now! Given that we have no reason to believe that Vox spoke Korean in those years, that's a long time for a language to become almost impossible to follow. Heck, we see that with internet slang every day.
Further, if Vox died in the 50s as is, I believe, at least Word of God established, this means that he died either immediately prior to, during, or after the bulk of the Korean War (which is technically still ongoing, aha)—this means that the Korean he would have known is not only 70 years old, it's from a country that no longer exists as we know it.
Since the Korean peninsula was split into the Democratic People's Republic of Korea (DPRK / North Korea) and the Republic of Korea (ROK / South Korea) in 1945, and then signed the Korean Armistice Agreement in 1953, the two cultures have been relatively isolated from each other and developed along completely different tracks—language included. These differences can be seen in both the vocabulary and the cadence of how they speak. In the second video from about 13 years ago, the narrator is speaking Seoul broadcasting dialect, and the woman in the video is speaking the most common North Korean broadcasting dialect. I'm not sure if non-Korean people can hear it, but the difference in cadence and emphasis is quite distinct to a Korean ear.
This isn't even touching upon the fact that even in modern North/South Korea, regional dialects can be incredibly difficult for people from other regions to understand! At least in South Korea, this forces everyone from other provinces to learn the Seoul Dialect, which is the one that everyone worldwide is probably most familiar with through kmedia, ahaha. I won't get too into this, but this video gives a decent snapshot of how different they can be.
Then there is, of course, social context. When Lace and I headcanon Vox as wasian, we're doing it within the context of actual US history, meaning that we take into consideration period-typical discrimination. Given the historical tumult of the 1930s onwards for Asian-Americans, it makes sense to us that Vincent, in his desire to assimilate into White Americana, would forgo his Korean heritage as much as possible. This seems likely to us, given that it's a pattern of behavior we see continue even through the 70s, 80s, 90s, and even into the 21st Century, when the stakes for East Asians weren't nearly so high (though discrimination continues on in its new forms).
It's a common story in any immigrant community, but very much so in Asian-American communities, that the children of first-generation immigrants either lose touch with, or never learn, their parent's native language, to the point where there are countless songs and essays written about it (Lace: It happened to me ^_T), but here's one from a Korean person who gives some context as to why, although I emphasize that this is one person's experience and that other people's—including mine—are very different. (And as a linguist, ignore where they say Korean is an altaic language!! That is a widely rejected theory.)
So given all this, in our headcanon, whatever Korean Vincent would have known is limited to what his mother would have used with him, which was an older form of Korean to begin with, and a language he has had little contact with in seven decades. Given the time period, he was disincentivized from using the language, and most likely he went all-in on a mastery of English that let him become the broadcasting star we know him to be. Given internalized racism, there's likely little reason or motive for him to pursue a connection with Korean culture post-death either.
God all this and I didn't actually answer your question ahaha, but what I'm getting at is, I think that he probably can't understand spoken Korean, especially modern spoken Korean. Maybe an old nostalgic word here or there, buried deep in his memory, but even that depends on what dialect is being spoken! I personally think if he wants to understand Korean, forgoing any fantastical intervention, he'll practically be starting from scratch.
Lace: I think prior to losing his Overlord status (I think) in the S2 finale, he could download language packs, though…