A Vault ID is a locator, not a username
No email, no account record, no profile. What a Vault ID actually is and what it deliberately isn't.
It addresses an object. It does not describe a person.
A Vault ID is how SEES finds your encrypted blob. That is the whole job. It is not a username, because there is no user record behind it; not an account, because there is nothing to sign into; and not an identity, because nothing personal is attached to it.
No email means no join key.
Most privacy leaks are not decryption — they are correlation. An email address links an account to a breach dump, a mailing list, a support ticket. SEES never collects one, so there is no field to match your vault against anything else.
Pick one that isn't already your handle.
Because a Vault ID is a public locator, reusing the handle you post under elsewhere hands back exactly the correlation the design avoids. It costs nothing to pick something unrelated, and it is the one privacy decision here that is entirely yours to make.