I've been gone from Citrix quite a long time but I'm pretty sure you mean Citrix MCS (Machine Creation Services). Citrix supports this VDI VM deployment method along with a PXE boot based technology (PVS) for dynamically deploying VDI instances. If memory serves correctly the MCS "master images" are standard issue VM's which we can back up w/o any issues. The MCS worker images (VDA's in Citrix nomenclature) are special VM's (details escape me) which may not be supported however this generally isn't an issue since Citrix maintains methods of storing user profile data centrally and can reconstitute a user VDI desktop dynamically. Other infrastructure elements ("Delivery Controllers", etc.) can be backed up with Veeam. We do get questions regarding VDI infrastructure protection from time to time but don't have any specific best practices.