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When I am using a VPN in a foreign network e.g. at my workplace or in an open W-LAN of a coffee shop, is it possible for the network admin to bypass my protection from the VPN?
How does it look like for the admin if I am using a VPN in his/her network?
They would see the initial communication to setup the VPN tunnel and they would see the source and destination communication to maintain the tunnel. But all data within the tunnel, (IE where you were routed out the other end of the VPN) would be completely hidden.
Example: VPN from A(laptop) to B(VPN device) is visible. The fact that you access stackexchange.com across the VPN is hidden.
Keep in mind, VPN can allow "split-tunnel" which ONLY routes traffic destined for the subnets on the other end of the VPN to go through the VPN tunnel. All other traffic would go across your coffee shop network in their plain view. To be secure, split tunneling must be disabled. Ref: https://en.wikipedia.org/wiki/Split_tunneling
Edit: Some additional thoughts...if at work (not recommended) your network administrator could have a proxy/NextGenFirewall that actually intercepts encrypted connections and acts at a Man-In-The-Middle to decrypt, inspect, and reencrypt as the traffic goes through. Doubtful this would happen other places like hotels/coffee shops.
Assuming that your VPN client is configured to send all traffic over the VPN (I've used corporate VPNs that only forward internal domains), your network administrator will see:
An additional note on "all traffic" is that often people forget to forward DNS requests through their VPN; if you don't, then the netadmin will also know which sites you're visiting (but not the full url).
