If they stop currencies and upgrade materials from being placed inside the vault I'd probably stop playing ASAP. There's no way that would be fair.
Also people could just do friends favors and allow them to recover their Gamertag on another console to have access to their vault
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Dude, make a separate vault and make sure in the <CONSOLE WIDE VAULT> you can't put Currencies or Upgrade Materials. If you could put them in the console wide vault people would make a new profile, level it all the way to 20, put there old gear in the vault and take it out on the new character than get the 9 coins for completing the strikes. that equates to 40+ coins per week they could potentially obtain and that would be game breaking. And as for part 2 of what you said, if people want to do that they can do that if they so choose but you would have to have quite a lot of trust in someone and make sure they don't steal your details. So in the end they are basically family if you trust them that much with your personal information.