Nothing says you have to stick with vinyl for these one off bags. You can use the heaviest construction poly film that you can find. I wouldn't use black, because I always want to see through it.
For your project, if your shorthanded or just have one helper, I'd set my form up on saw horses or a work table where I had the bottom half of the bag laying on top, then start with the inside face laying on top of a sheet of post-form laminate for a platen, then thin plastic film, then face veneer.
I use the 1-1 mix ratio epoxy from FGCI which I then mix colloidal silica into to get the viscosity up a little thicker than latex paint. For a project this size, I like to measure out the resin into separate mixing cups, then mix in the silica ahead of mixing parts A and B, unless you have a third person just mixing while you and another are rolling out the epoxy.
Rather than pouring the epoxy into a pan, I like to just pour out a puddle onto the substrate, spread it out with a scrap of wood, then work it outward with the roller. By mixing small batches it never spends enough time in a pot with depth to start heating up, so there is plenty of open glue time unless it's really hot in your shop. Unlike most other adhesives, temperature is your friend or foe. Cooler will add more time.
As you spread glue on one side of the bending ply core sheets, just flip them over onto the growing stack until they are all built up right in place on the form.
Rather using the rope or string caulk or butyl roll like Jared suggested, while that will work, you can just use the butyl in the caulking gun tube. it's a little thinner, and very sticky, so run a heavy bead around the outside, then bring the top sheet over and mash them together.
Most tutorials show using a bleeder fabric over the top of the pack, but I usually just use the thin plastic drop cloth from the hardware store between the platen and face. In this case post-form laminate would be about the right stiffness. I've even used it as the second ply under the face, since your probably going to have staggered seams in your bendy stuff. Sand the face with really coarse abrasive to give the epoxy tooth to grab onto.
When you make your form, make the ends run a bit longer, so you have some leverage past the bend. You can never bend any wood all the way to the end. It takes at least 8 times the thickness longer.
While you may be tempted to use 3/8" bending ply, I'd consider using 1/4" since it would probably be about 50% easier to bend, and less prone to want to spring back a little on the very end when you trim back to the spring line at the curve when your done.
I usually run around the whole perimeter with something like a cheap ski rope or parachute cord, then connect to the hose. I tape an old shirt sleeve onto the end of the hose, and make a big wad or caulk where it enters the bag. As your drawing the vacuum, once all the extra air is out of the bag, what your doing, is slowly stretching the air within the wood in the core, which will travel out to the edges faster than the face, especially if you've used plastic lam for the platen and maybe second ply of your sandwich.
Oh, one last thing, when building the form for the bag outside, you will need to clamp some straight 2x4s along the edges of the straight part to keep it from buckling up, so maybe cut some holes for the clamps.
When actually making the final bend with everything epoxied, take it nice and slow. The inside face is going to be getting compressed end to end, while the outside it going to be getting stretched. It may need to slide sideways about 2", so take it slow and let it flow.
Here is a link to the activator that I buy, and use with their standard resin. You can probably get by with the 1/2 gallon kit, but once you use it, you probably will want to always have some around, and the price is about half what some of the other consumer brands charge.