May a Windows Server (2000 or higher) volume license be used collectively by unassociated companies via separated VMs?

A volume license may not be used collectively by unassociated companies via separated VMs because scenario involves commercial hosting which needs to be covered by a Service Provider License Agreement (SPLA).
Only the licensee may use and access the licenses from his volume licensing contract. Such volume licenses may not be provided to unassociated companies.

Microsoft customers may install their software on third party devices as long as:

• it is a dedicated environment, that is, the devices are exclusively meant for and used by the respective customer
• the server is licensed by the customer himself

In this case, a collective use of a server via separated VMs is also not allowed and would require a SPLA.

Source: Microsoft Product Terms, May 2017