I'm working on a similar small pack, I actually just finished one, but the straps are off by a touch so I think I'm going to redo it, I'm using vx materials too, here's what I found:
25 liters is just right for up to 3 days/2 nights, so you got that one correctly.
vx03 is really thin, vx07 is not really strong enough to sew stuff onto unless you reinforce it a bit, I used vx07 for the sides/front and vx21 for the back/bottom, that is just about right if you want the pack to last more than a few trips. There is very little square yardage of the vx07/vx21 total, so the weight just is not really that big a deal, and I'd much rather have a pack that I don't have to worry about literally just touching a branch/twig/chapparel in the wrong way to get a hole, if you want real weight savings and strength, get the $30 a yard hybrid cuben that comes in at around 3.5oz yd and has a 50d polyester exterior, which is stronger than vx03 by far, and only costs about 12 or so a yard more. zpacks sells the hybrid cuben.
A while ago I used a tx03 for side panels on a test pack, and that was literally showing daylight through it via small pin hole breaks in the material before I even had finished the pack, I'd never use that or anything else that thin again for sides/front, maybe for a roll top or something, but even there I'm not sure.
I really like the vx materials because they solve one of the oldest issues with coated nylon, delamination over time, by putting a sheet of plastic inside it, and protecting that from contact with a thin sheet of polyester, a perfect design in my opinion.
Oxford is good, I use it for pockets and strap tops, it's fine, it's basically the top layer of vx21, 200d nylon that is, it's easy to work with and strong enough if you are careful.
there are two main types of mesh, one is much heavier than the other, but look similar, unless you go very fine. I'm referring to the mesh type that is sort of like fishnet material. Be careful of stuff that is too light.
I use regular nylon 3mm cord from I think diygearsupply, shock cord can't be tightened like nylon cord can. I used the same zig zag, with a lineloc 3 for the adjuster part, works well.
1oz / yd nylon is fine for inner pockets, I've used that, it's delicate but fine inside a pack.
The shoulder straps are fine with 3/4" nylon webbing, note there are regular and coarse types of webbing, the regular holds better, but not all companies online tell you which they sell. I believe zpacks and diygearsupply sell the regular more fine type.
zimmerbuilt has excellent hardware, there's a pretty big difference in plastic hardware, his stuff is good, well sourced. Look for more rounded ladderlock strap adjusters if you can find them, there's a sort of basic cheaper kind that is sort of squarish that I don't think is as good.
If you use a hipbelt, a 1" buckle that adjusts on both sides is nice.
With real pockets, not just webbing and some other doodads, this pack weighs 11 oz, and will carry I think 3 days trip as long as my trailhead weight is under 15 pounds. I did not use a hipbelt, nor do I see any particular benefit to using one in this size, I get no detectable side to side movement when walking. If I need to carry more water when starting out I'll use a water bottle holder on my belt to keep that weight off my shoulders.
Hint: if you want your final pack to succeed, get cheap nylon and foam and create the basic body template and strap template and sew it up quickly, including the top and closure system, then walk around in it for 2 hours. I swore after my last pack I would do that next time, and of course I didn't since I'm lazy and now have a very nice looking prototype pack that is a bit uncomfortable under the arm pits because the straps are just a touch off. Don't add any doodads to the prototype, all you are doing is testing your templates and pattern for actual functionality.
I found that a 3" deep front pocket is far better and more versatile than a 2" deep one, and there is almost no extra weight. A rolltop of about 8" height gives you a bit of extra room for fluffy stuff if needed but usually will just be rolled up and also costs almost no extra weight. Y strap over the top attached under shoulder straps can pull the straps back a bit to maintain the pack form, not a huge deal but it's something.