I read the sticky but wanted to make sure -

If I purchase item vault slots is the vault available to all the characters on my account or just the character I was on when I purchased them ?

all

Yes it is available to all characters, but be aware if the vault runs out, then you can only get down to the items if you spend again fg to the vault.

Wow, that would totally suck if you had to buy a vault for every character

The whole purpose of the vault, for me at least, is so that I can keep all my climbing gear in there then not have to change chars to get gear as I climb

For all the Characters in the same Core.
Sc and Hc Vaults have different vaults.

this