Since you say "changes made to the VM don't matter" then you can do what I do which is
- Put all the VMs in one folder
- Exclude the folder with the VMs from Time Machine
- Manually back up the VM folder. I keep a copy offsite on a portable RAID 1 drive.
Letting Time Machine "do its thing" is a bad idea, as it will make a new copy every it does a backup, which is generally around once an hour. You will eat up all your storage space very quickly and it will be pointless anyway.
You could "backup the VM using time machine once, then exclude the folder that contains all the VMs" but the Time Machine interface is not the best for retrieving your saved copy in that situation, plus Time Machine will delete old files if it needs to in order to save space and you don't know when it will delete your one and only backup copy of your VM.
By doing a manual backup of your VMs you can also refresh them once in a while (say when new guest OS updates are applied) much more easily than with a Time Machine solution.
Snapshots affect performance quite a bit unless the snapshot and the virtual disk is on a high-performance solid-state drive. It is worth considering if you need regular backups of ongoing work on the VM, but since you say you don't care about changes to the VM then snapshots are best avoided altogether.