If you find the Quicken vault password not working, you need not sit idle. Firstly, you have to store the password through a one-step update. You can perform this here:

Navigate to Tools > One-Step Update.
Now submit the password in the given box and press the Save Checkbox.
Hit Update Now to finish the One-Step Update.
If the above-described actions don't fix the error, it can happen because the bank account sign-in password may contain a special symbol that clashes with the Quicken feature for download transactions.
You need to do the following activities then: -

Kindly sign in to your linked bank account website.
Please modify your password, which should not contain special signs (! #$%^&*)
Please open the Quicken software, head to the Tools Section, go to the Password Vault tab, and select Add/Edit Passwords.
Now, hit on the account in the Password Vault.
Select the Change password for the account.
Now, provide the same password you use to log in to the bank account, hit the change, and you're done.
This will solve your Quicken Vault Password Not Working error. But if it doesn't fix the trouble, then communicate with the Quicken customer service team.
More Info- https://resetquickenpassword.com/recover-quicken-file-password/
More Info- https://resetquickenpassword.com/reset-quicken-username-and-password/