Pretty much any resin applied as a thin layer is going to remain slightly flexible, and on their own, each type of resin is going to have pretty similar properties. One exception to this would be resins that have glass fibers premixed. A product is sold under the Bondo label that falls under this category and can be found in some hardware stores. The downside of them is that they are hard to apply in a smooth layer, and they can't really be sanded because the glass tears apart sandpaper. So you have to first apply the glass-bearing resin as best as you can, and then apply another resin overtop that will either self-level or is sandable (since you're dealing with curved pieces, you'd want the later). So yeah, bondo glass putty followed by bondo autobody filler, followed by sanding smooth. These products should be used outside or in an open garage.
If you don't mind spending the money or special ordering, you can use similar products that are in the polyurethane category. Polyurethane is great to work with because it has no fumes, it's safe to use inside, and it cures quickly and strong. Smooth-On plasti-paste II cures nice and rigid, and Smooth-On Shell-Shock gives an excellent smooth surface (with skill, you can apply it so smooth you don't even need to sand it.)
But yeah, since you're talking about functional sparring armor, this is one of the rare situations where I do recommend going with fiberglassing. The above mentioned techniques might not be able to handle the abuse (and in the long-run almost certainly will not).
You should practice applying a small batch or two of resin and fiberglass to some random hunks of foam. This is super important because mistakes can be hard to fix.
If you use the polyester resin that's sold in the hardware stores and such, you need to work outside on a dry day when the temperature is moderate. If you don't mind spending some extra money, you can go to a marine supply place and pick up some epoxy resin. It is much lower odor and can be used safely inside.
If the armor is really just simple curves, then you should be fine with the woven fiberglass fabric, as opposed to the Chopped Strand Matt. The fabric is less messy to work with, but harder to push into fine details.
After applying the layers of cloth (3 layers is best, but 2 is livable), use a plastic scraper and a bit of pressure to scrape off as much excess resin as you can (this is what allows the fiberglass layer to be both strong and thin).
Once the scraped surface as cured to slightly tacky, you want to apply a thicker layer to smooth out that fabric texture. You can either buy expensive gel-coat from a marine supply store, or you can just thicken the resin you already have. You thicken it by stirring in this powdery stuff called Cabosil, aka fumed silica, aka colloidal silica. Wear a dust mask while using this product, because it floats everywhere. If you can't get ahold of that, then talcum powder can be used in a pinch. That can be bought at most pharmacies. You mix the powder in until it reaches a mayonnaise consistency and then add the catalyst. It's difficult to apply thick and smooth in one go; the best way is to apply the first layer with a squeedgy so it settles into the low spots, and then let that cure to slightly tacky and apply a final layer with a wide soft-bristled brush; that, or apply it thick in a single layer with a cheap brush, not caring about the brushstrokes, and once cured, just sand it smooth starting at 60 grit and moving up to 400.
The top layer will remain a little tacky; don't worry about this, just scuff it with a scouring bad (the green ones from the kitchen) and apply your vinyl. If you have the time and want to minimize the chemical smell, let your pieces sit under a sunny window for about a week.
Hope this helps!