The bug

Vaults are not unlocked by using a key item with a non-default max stack size.

To reproduce

• Execute the following commands:

  /setblock ~1 ~ ~ vault{config: {key_item:{count: 1, id:"minecraft:dirt"}}}
  /give @p dirt 2
  

• Use the dirt block on the vault and notice how it is unlocked.
• Repeat the first command.
• /give @p dirt[minecraft:max_stack_size=2] 2
• Try unlocking the vault using the given dirt.

Observed result

The vault is not unlocked.