If you use a VPN service you should be considering a paid service.
It really only makes sense - and even at that - still requires care in choosing.

While a VPN protects your internet traffic from others, it IS (naturally) visible to the VPN provider.
To what extent depends on what you are doing but it is still worthy to know as non-encrypted traffic can be seen.

Read this article for a non-technical commentary..

You Should Not Trust Free VPNs