The last ability of Storm the Vault doesn't trigger if you don't control five or more artifacts as your end step begins. If it does trigger but you don't control five or more artifacts as it resolves, it does nothing.

Storm the Vault's first ability can trigger more than once in a turn if creatures you control deal combat damage at different times in a turn (most likely because one or more has first strike) or if creatures you control deal combat damage to more than one player at once.

In a Two-Headed Giant game, if you control more than one creature that can attack, you attack different opponents so that Storm the Vault's first ability triggers twice.