Old Virto licensing system is tightly connected with hardware configuration of server machine (even if server machine is virtual). In case when hardware configuration changes significantly license can be reset and license reactivation is required.
In case of virtual server(s) we recommend to fix hardware configuration (CPU speed, HDD/Memory sizes, MAC address(es) of network adapter(s), etc.) of the server(s) giving some more resources than usually needed for normal work.
All latest versions of Virto products for SharePoint 2010/2013 use new licensing system which does not have such issue. We highly recommend to update your versions of the products for avoiding this issue in future.