Can use of a vpn cause loss of wifi connectivity?

Loss of Internet connectivity over Wifi? Absolutely.

Loss of Wifi connectivity to only your router? Probably not.

Just to make sure you have not fallen for the marketing tactics of most VPN's claiming they are offering Security and Privacy, actually the opposite is true in many cases, review the information provided here:

• https://gist.github.com/joepie91/5a9909939e6ce7d09e29
• https://overengineer.dev/blog/2019/04/08/very-precarious-narrative/

As for legality of VPN use, in most regions they are not illegal. Using a VPN to access content that is not available in your region may involve copyright infringement, which can be a legal issue.

As for the issue you are having with the VPN, testing is simple. If you have internet connectivity without it installed and installing it results in failed connectivity, you have found your problem. In that case contact the VPN provider to a resolution with the OS version on your computer. It is not unusual for developers to drop support of older OS versions to concentrate on the last 3 releases, but that is something only the Developer of the VPN could answer for you or provide any updates if they choose to do that.

Wi-Fi connectivity is independent of Internet connectivity.

The distinction is relevant because the direct answer to your question "Can use of a vpn cause loss of wifi connectivity?" is no.

Two separate problems, two separate solutions:

If you can't connect to a Wi-Fi network on Mac - Apple Support

If your Mac isn't connecting to the internet over Wi-Fi - Apple Support

The latter expounds upon VPNs. Specifically, If your device has network connectivity issues, check for VPN and other third-party security software - Apple Support.

The short answer is don't use them.

I thought security was the point of vpn's ...

It really isn't, marketing claims to the contrary notwithstanding.

See VPN: What you need to know - Apple Community

Yes, your VPN app, and/or the VPN service itself, could interfere with internet connectivity regardless of whether it's via WiFi, wired ethernet, hotspot or other. On that point the technician was correct.

VPNs are not illegal, at least not in the USA. But off the shelf, free, even paid VPNs on the market are not what they purport to be. Most claim a high degree of privacy & security when in fact they are collecting your activity data and in some cases found to be reselling it. There have been a notable number of court cases over this. Second, just because a VPN asserts that it is "located" outside the USA does not mean that it is immune from law enforcement action. And as for getting around geo-location blocking, many providers have figured out how to detect a VPN run-around and block you anyway. Point is, VPNs are not all they're cracked up to be.

The only really secure VPNs are corporate VPNs that employees use to access their work environment.