@thomas-pondant,

This can be because of many things.

It could be the Power of the wireless access point. It could be the wireless nic on the connecting device. It could be additional EM field interference from Microwaves, TVs, Electrical bleed from fluorescent lighting...etc. It could even be too many devices plugged into the same electrical circuit or that circuit having a common ground.
In other words, it's only by the process of elimination that you can isolate. There could be antenna issues itself. The VPN slows down the traffic because it has to build up the tunnel, filter, etc but it shouldn't influence the signal strength.

This is a layer 1 troubleshooting issue.

@thomas-pondant,

I'm not sure then I was clear then in the recording. The Wireless router is actually a combo device (Router, Switch and a Wireless Access Point). The only part of the Wireless Router that acts as a Layer 1 device is the Wireless Access Point. It operates using CSMA/CA and is half-duplex device.

All wireless access points act as a hub. If you device has one antenna, then only a single device accesses that antenna at any one time. Even if your device has antennae, only one connection per antenna at any one time, regardless of SOHO or production level devices.

I would like to echo the thoughts of many in this thread that a concise list of PowerShell commands to study for the exam would be very helpful. Even if you cannot teach a whole class on it, just a list of the commands across each of the 70-4xx courses would make it a lot easier to study for the exams.