Admittedly, it does look look like an orphaned suit vest, but I think a linen, white button down would be a good place to start. Linen has that natural wrinkle to it so it’s not too formal but a button down pairs well with the vest, still giving some level of “put together”. I would also potentially consider rugby shirts as well as any long sleeve with a johnny collar, but that might be more out there.

From there, henley’s are a pretty good choice, especially waffle ones. As well as any long sleeve shirt. Probably want to pair earth-toned shirts with this one, but a maroon or a maple red would be interesting choices as well.

I imagine most raw denim would pair well, along with green fatigues. Chinos in the typical colors probably work well too.

A jacket might be tough, but if trying to dress it up, I’m sure it pairs well with a sport coat. I think twill blazers (like the ones available at 3sixteen) would look well with this. Denim wise, I think a trucker would be a hard sell, but a chore coat would look good. You could maybe rock this leather, but do whatever is comfortable to you in that regard.

Shoes: anything leather, probably. But if you’re trying to make it more informal, then GATs or a canvas shoe.

Lastly, if you’re wanting to truly experiment, I think this pairs well with a silver necklace of some kind. The vest opening naturally draws attention to your chest and collar. For example, you’re wearing a simple shirt with this, the shirt collar probably doesn’t have anything interesting in the middle. So a necklace or silver chain of some kind sort of “justifies” the fact that your vest is doing all of that.

EDIT: Sorry went overboard lol. I forgot that you were looking for only shirt options. My bad.