The vMX100 firmware upgrades are unlike regular physical MX firmware upgrades. vMX100s cannot run firmware lower than 14.18. For any version 14.18 or above, you can schedule upgrade like a normal MX and it will upgrade successfully.
If you have a vMX network running any firmware below 14-18, the vMX will continue to work just fine. Let me know if you have any further questions.