This plugin allows your WordPress users to authenticate and login to Vanilla forums like Facebook Connect.
Yes, you can set Vanilla to only use SSO plugins and disable all other methods. However, you cannot remove the login button which pop's up the SSO WordPress login link as well as HTML that will NOT work if a user types there username and password. They MUST use the jsConnect login URL.
Yes and no, If someone logs on from the forums they will be redirected and logged in automatically. If they are already logged in to WordPress you will need to send them thru the special jsConnect URL, otherwise they will not be logged in just by hitting the landing page of Vanilla.
No, This plugin is similar to Facebook Connect. It only authenticates WordPress users in Vanilla. That's it.
jsConnect is maintained by a Vanilla co-founder by the name of Todd and in now favored over the older ProxyConnect addon.
You can, but I would not recommend it. Each Vanilla user that was created with the old hashes will be required to re-link their accounts. You should enable the new "Protect Data" option and copy and paste them in a safe place in case you need to restore your settings.