What is the difference between “/private/var/vm” and “/vm”?
This happens because of a change that happened within the High Sierra versions. It is unknown whether this is a bug, or simply a change of hearts at Apple, but with High Sierra the location of the swapfile was changed from /private/var/vm to /vm, and then in a later update to High Sierra moved back to /privat/var/vm.

The best way to proceed is to ensure that you have upgraded to the latest High Sierra version. If the extraneous swapfile in /vm/swapfile0 still exists and takes up disk space, then boot up in Recovery Mode, use Disk Utility to mount the main volume, and start a Terminal and run: (replace TheVolumeName with the name of the main volume)

rm /Volumes/TheVolumeName/vm/swapfile0


Then you can reboot afterwards.