so modern armor is an interesting topic. kevlar is plastic, typically made into thread. solid block kevlar has been used in the past, but isn’t really used anymore. it has properties similar to silk, so one could use silk to get similar protection. essentially it is a high tensile strength thread, which when used in layers provides enough protection to ‘catch’ a bullet. it doesn’t work by using mass to stop the bullet, thus you tend to have a lot of kinetic carry through. it is flexible, so it doesn’t disperse the energy very well, though as you know with gambeson more layers means less flexibility and thus more dispersion. most vests are thin, and either use plates for reinforcement or not. the thinnest ones are the ones you see in Hollywood that are really thin and look like clothes, or are fitted under clothing. the thicker ones don’t fit under clothes very well. the thicker ones tend to have extra padding and pockets for plates.
as for plates, they are either steel or ceramic. steel can be a few different thicknesses, with the thickest being useful for mild rifle fire before being penetrated. ceramic is lighter, though it also is thicker than the steel for equivalent protection. they aren’t made as light duty as steel, as they need the mass to absorb the energy, thinner plates would be much worse than steel and can’t really do low end protection. they are made for light rifle fire and sustained rifle fire (best protection currently). while they do fragment instead of stretching, the thicker versions stay together much better than its thinner counterpart and thus breaks apart much slower. the ceramics are made into helmets and other forms of protection as well, just like steel. only the best equipped forces use ceramic plates for its forces, and most forgo any protection.
so as far as stab wounds go the thicker kevlar vests will protect you like a thin gambeson, unless it has plates, in which in the area of the plate it is very good prtoection. some vests will use the kevlar as a way of holding plastic plates in place just for stabs, though the kevlar tend to be rather thin and not useful, though like any cloth it can be made many different ways.
so in medeival times it is possible to get kevlar like protection by using a silk gambeson, which plenty of forces in the far east did, and it works well against arrows. the reason stabs do better is the mass of the person behind the stab, arrows however lack that mass and rely on force in a small area and pushing the threads out of the way, which as you know from gambesons only works so well, and enough layers will defeat such. i forget how many they told us the military issue vests were when we were issued them, but it was more than 24, as that is standard for level 2 vests and these were considered better in areas that there weren’t plates. so what ever it takes for a gambeson to stop a stab or a given arrow hit then modern kevlar could do it in less, how much less is up for debate though.
of course there are some new materials and designs out now that change this a bit, some (like dragon scale) are lighter, and some (like special composite ceramics) are tougher.