Hello!
Actually the behavior for vMX is differently. Here is a quote from the public documentation:
"You can download the vMX software BASE application package with 1 Mbps bandwidth and evaluate it without a license."
After the evaluation period is expired, vMX does not stop working, it is just reduced to a 1 Mbps throughput limit, but you can use the features of the BASE license (IP routing with 256,000 routes in the forwarding table) without any interruption further.
Please note that this is just the technical aspect, the product itself is still not licensed and may violate license policies.
Here you can find the whole documentation:
https://www.juniper.net/documentation/en_US/release-independent/licensing/topics/topic-map/vmx-licensing.html