Eat some pies, wear a non-covert one under a big shirt. You pay a _lot_ for the extra-thin covert ones.
Also they're no bloody use. If you're in a situation where such a thing is likely to be worn (doorstaff maybe), then your general demeanour intimidates many, drunken students are a nuisance and little more, so your largest actual threat is from the serious nutter. They aren't going to stab you in the vest, they'll aim somewhere more effective, just in case you are wearing one.