Use VPN only on wifi (or not-trusted wifi) ?

[lolex]

Hey there,

I am looking for a way to use VPN on Android only while being on public wifi networks or wifi networks that are not in a "trust list". Why ?

It will save VPN traffic.
It will safe battery.
It could increase network speed on certrain wifi networks.

I don't need VPN for security reasons on 3G/4G and on my home network. Is there any way to do this?

Android 5.1.1 and Nexus 6.

Thanks !