MBP 13" 2015 + high sierra with external monitor : wifi being spotty and dropping service irregularly
External monitors can emit signals that interfere with WiFi signals. This is commonly due to insufficient shielding within the monitor. I would complain to ViewSonic - they might know about a design deficiency for this particular monitor, and have a solution or rebate at hand.

In order to try and fix this yourself, the most common way to solve this is simply to move your WiFi to a different set of frequencies, and hope that the monitor is not producing interference with that set of frequencies. It is usually hard for home users to shield the monitor properly without it being an ugly hack.

In essence you would need to access the settings on your WiFi access point and move it to a different channel. For example if you're using channel 11 now, try using channel 6 instead. You might need a few tries to find a sweet spot.

You can also switch from 2.4 Ghz WiFi to 5 Ghz WiFi if you have that available.