I don't think you can just use jackets as flak vests. Have you run this through the app team?
Remember that even if you can mod the jacket as a flak vest you have to follow the CRL, which specifically states that you require a flight suit. The CRL is made based on visual references so that all costumes are consistent and fit in the era they are designed for. Not to mention that modding a jacket as a flak vest would be a considerable effort and would require a lot of work. Frankly it doesn't seem like it's worth it.
You can make a two-part flight suit if you want, but you can't just use a regular shirt you picked up at a local store. It has to be something that looks in-universe and, again, that complies with the CRL and the overall SW and Mandalorian aesthetic. Furthermore it has to give the appearance of a single-piece flight suit.
Here's a link to the
Costume Requirementes, make sure your ideas are in line with them and you shouldn't have any problem whatsoever. Hope this helps!