Edit (Update/Correction):

It looks like I was very likely wrong (a good thing in this case!).

According to this entry in the official Bitwarden Help Center bitwarden[.]net is official too!

Its still weired that the login failed multiple times and bitwarden[.]net doesn't redirect to the homepage, as the other official domains do, and so on.

Nonetheless, all seems to be fine, gladly. Thanks for your input.

I've tried to log into Bitwarden and used vault.bitwarden[.]net, it failed. Also, this page is the only one on this domain (nothing on bitwarden[.]net) and the SSL Certificate differs.

vault.bitwarden.com (and bitwarden.com) behaves as expected.

I think I've actually used a phishing site. 2FA probably safed me pretty hard. Changed creds.