If you have installed the latest April 2018 update version 1803 for Windows 10, the installation of the vJoy driver might fail and the installer will say installation failed. It doesn't seem to happen for everyone, however, so we're still interested in seeing how many are affected. So please submit a ticket if you can't install vJoy or if an existing vJoy driver has issues after upgrading to Windows 10 version 1803.
vJoy is a third party application so unfortunately, we can't provide a fix ourselves. The issue has been logged for the vJoy project here.
For the time being, you can downgrade to Windows 10 v1709 to make it work.
UPDATE: The creator of vJoy have released a new update that fixes this issue (Build 39). It can be found here.
Before installing this version of vJoy make sure the previous configuration (if causes problems) is removed: Run vJoyConfig -r then install this version of vJoy.
OR run the attached bat file.