Screen Sharing (Mojave 10.14.6): I see the login screen but mouse & keyboard not working

The Remote Management and Screen Sharing services are two different, but similar approaches to accessing a Mac remotely. The former requires using the Apple Remote Desktop (ARD) app to access the remote device; the latter does not. They cannot be enabled at the same time.

Which are you using, or want to use in this case?

Ok, then:

• On the "remote" Mac Pro:
  • Be sure that Remote Management is disabled
  • Have Screen Sharing enabled
  • Depending on how you want this computer to be accessed, choose the appropriate option for Computer Settings...
• On the iMac:
  • Be sure that Remote Management is disabled
  • Have Screen Sharing enabled

Since this will be done over a VPN connection, check to see if you are connected to that VPN as "split tunnel" vs. "full tunnel." You will want the latter preferably.

Ref: How to Screen Share With Another Mac's Desktop - Lifewire

You're very welcome! I'm glad you got it working.

Just to be sure, by "Screen Sharing," are you referring to the built-in macOS screen sharing feature or another third-party app? Since you mention a VPN, I assume that you are attempting to access the iMac Pro's screen from a Mac Pro at a remote location over a VPN connection ... correct?

How is the VPN tunnel being established? For example, your home's router provides VPN service, and you use a VPN client on the remote Mac Pro to establish the VPN connection? If so can you successfully ping devices on your home network from the remote location? Per chance to you have any firewall rules established on either the home router or the iMac or use any form of antivirus apps on either computer?