The short response to the question in your topic title is “yes”. I have discussed this in a previous post.
Yes, it stores a local copy of the encrypted vault. You need to make an actual copy of the vault (.\bitwarden-appdata\data.json in the installation location), because if you connect to the internet after 30-days of inactivity, Bitwarden will log you out, which wipes out the local vault copy.
Yes, it’s completely portable. Bitwarden Portable is intended to be installed on a USB stick that you can take from computer to computer. Also, you do not need to save it in an encrypted volume, because the vault is already encrypted. You don’t gain any security by encrypting it again, and doing so will require you to remember another password.
Your attachments are stored on Bitwarden’s servers, so they are not actually included in the portable vault (the vault has the URL links to where each attachment is stored in Bitwarden’s cloud, but it doesn’t include a copy of the attachments).
Nonetheless, I consider this a safe way to make backups (if you are careful about taking precautions to deal with login session expiration). As noted in the referenced post, it has benefits over backing up using various forms of exports.