Are you sure you did nothing in addition to steps 5-9? Because Vaultwarden has been adjusted to listen to localhost 127.0.0.1 only and not to local network anymore. Therefore you are not able to each it actually.
Which btw, is perfect for the configuration of a revers proxy 
Next step would be to create /etc/nginx/sites-dietpi/vaultwarden.conf and to adjust /etc/nginx/sites-available/default as describe on How to setup vaultwarden - #19 by Joulinar
That would be my perfect setup!
So i did that! So inserted that config into the file with my IP adress and uncommented /etc/nginx/sites-available/default
usually, you would need to use 127.0.0.1 if both Nginx as well as Vaultwarden running on same system. Because Vaultwarden is listen to that IP only.
Okay! Thanks for your help! 
I’m not an expert programmer. I configured nginx as a reverse proxy for adguard home, phpmyadmin, nextcloud and vaultwarden, following your precious advice. After uploading the vaultwarden configuration, I can no longer host a website. the web page gives me a 404 error.
If I uncomment the default file at the location / lines of the default file in the sites enabled folder, as suggested for the configuration for vaultwarden, the website is reachable but not the vaultwarden login page.
Sorry for my English.
Maybe you can share the configuration you are using. That would be helpful.
I use the configuration proposed by anarchy wisely.
this step disable host of web site .
This is going to remove configuration for root directory. Do you like to use a sub path or sub domain or how should the URL looks like for Vaultwarden??
How can I reach both the website and the vaultwarden login page?
you need to setup Vaultwarden to run with sub path and use this path for your proxy configuration.