Don’t have a VM?
Use Carbon Copy Cloner to make a bootable copy of your Sierra installation on an external drive, then upgrade your internal drive to High Sierra. This gives you the ability to start up in either system.
Start up your Mac using the external drive when you need to use Sierra. Note that this doesn’t work well with Time Machine, though, and Sierra throws up errors with APFS disks. Paragon’s APFS Retrofit Kit (free, bit.ly/papfskit) should help solve this.
Disable Time Machine in Sierra and disconnect other external drives.