In no goalkeeping category was VDS 'miles ahead' of De Gea. You're giving Edwin all the credit when Rio and Vidic had just as much a role to play in controlling and organizing that defense. I loved VDS but he was never the best keeper in the world at any time with us, and our success was down to that whole unit while De Gea has had seasons where he's dragged our defense through games single handed.
Btw, there's nothing overrated about DDG's 'flashy' saves, either. He doesn't make routine saves look flashy like some keepers do, he makes genuinely difficult saves and often makes even those look routine. There's also no contest between him and Edwin in 1 on 1s.