Will a VPN prevent a Mac from using AirDrop?

Amplifying a little on what others have said .. some VPNs do prevent certain local networking. Others might allow it. The VPN my employer was requiring for remote access prevented local networking (couldn’t even print to a local network printer). However an earlier setup allowed “SPLIT TUNNELING” and then it did work. But that was deemed a security risk and discontinued.

I would UNINSTALL completely the VPN as a troubleshooting step to see if now Airdrop works. It can always be reinstalled later.

It depends on the VPN and how it is configured. If local and ad-hoc traffic are blocked, then AirDrop will not work.

At work, for years the VPN, of the company I work for, forced ALL network traffic to the VPN, so the ad-hoc network connections for AirDrop were blocked.

Other VPN's that only redirect Internet traffic and leaves local traffic and ah-hoc traffic alone should allow AirDrop.

It shouldn't have any effect.

I have SurfShark to stop the police seeing what I'm up to on my M2 and M4 minis and have no problems sharing items either way.

On occasions it has stopped Mail connecting but turning it off has cured it.

Is that a siren I hear . . . .

It could. If that is the problem, you may need to remove the VPN entirely from your computer. Just turning it off is often insufficient.

You might want to consider why you're using a VPN. Is this something you have installed because your school or work requires it in order for you to access secured servers? Or, are you using it because you think it provides additional security? If the latter, you might want to review these articles:

VPN: What you need to know - Apple Community

Don't use VPNs

as others have stated, then it depend on how the vpn is setup and not necessarily in your end.

during covid work from home times, my company changed their vpn so peoples port 80 (www surfing) would not be included in our vpn tunnel, to put less pressure on the vpn server.

why do you wish to use vpn?

some companies pitch vpn as some security thing like firewall or virus scanner or the likes, but it's somewhat snake oil, and in terms of data security and privacy it gives the vpn server manager 100% control of all! your data on a silver platter