Most of the VPN providers contain firewall rules and kill switches that can stop such traffic leaks from occurring. Leviathan Security has stated that firewall rules within the apps can negate this attack.
The most vulnerable at the time are VPN providers being used on iOS "Apple" products because they prevent a full creation of a VPN Killswitch.
It is a POC and not one that could be implemented easily on a ISP or cell network. It would take some serious effort.