This article explains how offsite VLANs are created and goes over some limitations and best practices associated with them.
VLAN creation can only be done while setting up an offsite virtualization, as the VLAN will be created on the compute server for the VM. This server will then be the "Master Server" for the VLAN, and the VLAN will be shared as needed. Only one VLAN with a given IP scheme can exist on a master server (For example, if a VLAN with the subnet 192.168.2.0/24 already exists and in use, you will not be able to create it, and would need to either select a different networking scheme or reach out to Datto Technical Support for assistance).
Remove the old VLANs after your Disaster Recovery or DR Test scenario is completed. This helps keep those subnets available when needed. Otherwise, unless your virtualization occurred on a private node, multiple devices from multiple partners will be syncing and potentially virtualizing on the given offsite server. Removal of inactive VLANs ensures that commonly used subnets (such as 192.168.1.0/24 and 10.1.1.0/24) will remain available.