VPSes are just empty Linux servers with a command line, you have to install everything yourself and they have nothing to do with a plesk account. You'll need to install everything yourself. If you need Tomcat to host Java, you'll need to install tomcat. If you need a database server, you need to either install a database server on the VPS, or you could enable remote access on your plesk account's database and use that.
The specifications you purchase are the specifications you get, but you have to remember that the operating system will use ram and disk space, so actual usable resources are slightly less.
Users are only allowed to have one Plesk account, but can have one or more VPSes in addition to a Plesk account.