How can I get this weird stain out of my polo?

I have no idea how this formed. Only thing I can think of is that it formed when I was soaking the armpit areas in distilled vinegar to remove some anti perspirant stains. Those stains were removed, but then this one formed. I soaked this area in distilled vinegar for an hour and then washed, but it hasn’t gone away. I’ve only ever hang dried this, so it shouldn’t be set from heat. It’s a Kent Wang polo, which is always sold out these days, so I’d like to salvage it if possible. It’s much more noticeable in person than the pictures let on.