Here is a very quick guide how to use Argon2 hash for Vaultwarden. You can use different ways to enable access to admin page here but like I said, this is a very quick solution:
Go to https://argon2.online/
Enter a passphrase in Plain Text Input, click once on the Salt cogwheel and leave everything as default and click GENERATE HASH
Go to Vaultwarden Admin's Page >> General Settings and replace your current admin token in plain text with the generated hash value ($argon2i$v=19$m=16,t=2,p=1$YnJvYm1vSD...........)
Save and restart the vaultwarden container
To login to admin's page you must use your plain text value, not the hash
I hope you will find this very quick tutorial useful.