Sure, you can inject the UserSessionService in any java service code like you were showing. What was not working when you tried?
Once you get a hold of the ServerInfo array userSession.serversInfo you will have to select which vCenter you are interested in.
In the case of the PPA sample we use the serverGuid of the context object passed to the adapter, but if you are not passing any information to your service you need another way to select the server you want to talk to since the Web client may be connected to multiple vCenter servers.