How to prohibit traffic bypassing vpn (needed for one 1 out of 2 possible connections!)?

Question

VPN

Good day to all.
Introductory: there is a laptop with windows 8.1.
The laptop consumes (at different times) two possible "Internet":
1. via wi-fi - there are no questions about this, and no ban is needed.
2. through a 4g modem that emulates a local network connection, gateway ip 192.168.8.1; ip received by the laptop is 192.168.8.2
And there is a vpn (pptp) connection (to my home router). Purpose: to completely prohibit traffic through this very second option until a vpn connection is raised.
Is this even possible?
Vpn connection will rise only when working through connection No. 2 (modem).
ps no interference with the Internet through the first (wi-fi) connection is needed.

Answer 1

In the profile of the second connection, you prescribe a static IP, do not specify default gw, you prescribe only a static route to your VPN server, as a result, no one will be able to go anywhere until you raise the VPN.