Got any suggestions on where I could buy one with a "fitting?"
Your location (at least the State) will help with that.
Presume this is an "under street clothes" vest rather than an exposed vest, yes?
A "concealment" vest winds up being very hard to self-modify. Especially one meant to be "stab resistant" (it will not cut easily).
Also, that high fit up into the armpits is meant to protect your vitals from lateral attack, too. So, getting more "armpit hole" actually exposes more of your necessary arteries.
Further, have you made the needful investment in layering garments?
A ribbed undershirt can be critical for comfortable wear. And you want at least one per day worn (two are better). You want moisture wicking undershirts as well.
Now, if you mean an external vest, worn outboard of the street clothes, you still need moisture wicking shirts. But, you also have to cope with reduced mobility (plate carriers can be particular bad about this). Generally, you need to "train through the pain" with most of those.