As I understand it, the OTP factor only protects the login. While this is still good practice, it wouldn't help in the event the vaults are stolen.
Yes, 2FA/MFA only serves as access control, to limit who can retrieve a copy of your encrypted vault from the server (to then decrypt locally).
Like in the Lastpass scenario, if someone gets a hold of your vault from a server side backup (or compromise), then your access control is bypassed, and won't make your vault harder to decrypt.
Using MFA is definitely good practice though, as in normal circumstances an attacker will be trying to get to your vault without server side access.
One thing that is also worth mentioning for anyone nervous about their password security is that you can use a physical security key with the paid version of Bitwarden. I need to use a yubikey to log in to any new instance of Bitwarden and it's been working well.