Within Android it is not possible to access another application's data. But an app could watch the radio layer (watching wifi and cell traffic). It would be able to know some info. The sites you visit for example, though not the content if it was sent over https.

If I employ a vpn service that encrypts traffic does that mean all the apps see is me connecting to the vpn server, or are the urls/browsing content still fair game?

Firefox now has Private Window which does not allow access by anybody, and its on FFapp, hit the add page button, then you will see a purple pair of glasses, hit that and you are in Private Browsing. Not like any other PB window