So you want to do a thaumaturgic ritual to give an item mundane properties via magic? Okay, but keep in mind that the effects of magic are temporary. You may change lead into gold, but it'll turn back by sunrise. It takes a tremendous amount of force to change something's shape permanently.
Here's how I'd let you do it:
Take the craftsmanship roll required to make a suitable kevlar vest (it's not just sewing the cloth together, but carefully making sure that you don't have weak seams, getting the right number of layers, the right type of kevlar, etc).
Add +2 for the difficulty in finding the material (do you know what raw kevlar and then kevlar in its treated forms are like? How long do you want this to take? You'd have to know their properties pretty well to replicate it).
A kevlar vest takes years to come from prototype to final product. We'll say you have a prototype, so weeks to manufacture then (from chemical treatment to finished product). Add +1 for each step up on the time chart (we'll call it something you want to do in an afternoon).
So, really, we're looking at probably 3 (base difficulty)+2 (unfamiliar materials)+5 (done in an afternoon rather than over a few weeks) at a 10 shift ritual that would feasibly last until sunrise/sunset (call it an afternoon). Then, bump it up on the time chart to make it last longer.
So to make it last a season 7 steps.
17 difficulty ritual to have Armor 1 (basic kevlar vest is designed to prevent penetration of bullets, not stop them cold like ceramic plating and other more advanced and more cumbersome types of body armor). This is all fully extrapolated from RAW. Even if I were lenient (say, eliminating or reducing the time to make), you're still looking at something like 10-15 complexity).
Better to steal it or use an enchanted item.