you could start by using every word in the English dictionary that starts with that letter (would take you years)
On a mainframe from the 80s maybe.
The number of words is quite finite and the number of words in commonly used wordlists even more so. On the order of thousands maybe.
Given that they claim to know the starting letter, that should narrow it down to hundreds.
Even at multiple seconds per check that’d only be a few minutes.
The other wrench in this problem is that Bitwarden vaults are not readily able to be brute forced. I won’t go into the specifics, but passphrases are not stored in “plain text”, but rather in “hashes”, which is kind of like a “fingerprint” of a file in that every file has a unique “fingerprint”.
A simple hash does nothing to slow brute force. It’s the underlying mechanism to do any password verification at all and usually rather quick.
State of the art for master-passwords are PBKDFs such is argon2i which are basically a hash hashed again and that hashed again and so on such that you must do a high number of hash calculations in order to verify a password; each depending on the previous.
You choose the number of iterations in a way that is still relatively quick to do in human terms but rather lengthy in computer terms (hundreds of ms to a few seconds). Every time you enter the master pw your computer runs through this PBKDF and you probably don’t even notice.
This does indeed “slow down” brute force attacks a good bit in relative terms but in this case the difference is inconsequential in absolute terms.
Bitwarden won’t let you constantly slam your vault stored on their servers with brute-force password attempts.
I don’t know about BW limitations in this regard but depending on whether @WtfEvenIsExistence is still logged in on any of their devices, they might be irrelevant because you don’t need to interact with any of BW’s servers even once to crack your own password. BW works offline if you have logged in once which implies that the pubkey, salt and whatever else is required to verify the password and unlock the vault are available locally.