dude Eco, love ya ner vode but this isn't a good idea for a bunch of reasons. You need a sturdy fabric so it'll hold it's shape with that added weight of the plates and stretchy materials just don't do it. I personally don't like the UA because it pills eventually (that's those little balls of fluff that get stuck to materials) and the lightest fabric I would use for something that's going to have weight added i.e. mando plates, would be a twill or maybe a cotton that's been reinforced. basically you want to look for a fabric that you can starch, like a cotton, and that can hold the weight of the plates. t-shirt materials are considered light weight jersey knits and you want to stay away from any kind of knit when your looking for fabrics that are stiff, it's the same with the UA fabrics, they are a type of knit which is why they are so stretchy (also the fabric blend but I'm not getting into that)
doing things on the cheap is one thing, cutting corners is a whole 'nother matter.