There's a few minor problems with this.
First one being that you have to safely pass an Agility into a Dunsparce, as Salac Berry / Sticky Web won't give Dunsparce enough of a leg-up to outspeed everything it needs to. If you're going to go through all the trouble of passing +2 speed, it might as well be from a Shell Smash, at which point, you sit and ask yourself why you're SmashPassing to a Dunsparce and not something like Gallade or Xatu.
The second problem is that you have to be asleep in order to use the move you want. If Dunsparce switches in with the boosts, immediately uses Rest, and gets Snore to flinch the opponent both times.. well, it can't Rest again as it'll be at full HP.
Last problem is, even if this all works out perfectly, it's still only a 60% chance to flinch each turn. There's a reason why Jirachi is so good (well, a whole fuck ton of reasons, actually) but even on sets where the main goal is to ParaFlinch the opponent, Jirachi isn't a sitting duck if it fails to flinch that turn - something Dunsparce is sadly.
Assault Vest does indeed "stack", meaning it adds an additional 1.5x multiplier to SDef. The problem with the Pokemon you listed is (aside from maaaybe Regirock) is that they want to use their support movepools to help the team (that's their best niche), and they have no way to recover HP, so they are vulnerable to getting HP chipped away at. Oh, and Sandstorm isn't very viable in NU atm, and if it was, you would much rather be using your limited Sandstorm turns sweeping with Stoutland or Sandslash.
Anyway, hope this helped! I really enjoyed your post because it's the first time in my life I've ever seen someone call Rest "100% reliable recovery" :)
Welcome to Smogon!