There are different things.
DIOS MIOS (Lite) has an option for using a virtual memory card.
Devolution also has an option for virtual memory card.
These are backup loaders, and the Virtual memory card is enabled by not having a real Memory card in the slot, and can also be turned ON/OFF by using a backup loader.
DIOS MIOS (Lite) is also working with Official discs or burned discs (if you have a modchip).
DIOS MIOS (Lite) virtual memory card is not compatible with all games, and it's based on NMM previous work from the same author. (No More Memory)
If you want, you can install only NMM, but I guess you better install DIOS MIOS or DIOS MIOS Lite as it works with both ISO and Disc, and has virtual memory support.
If you only want the virtual memory support, just install NMM but there's no switch, it's always active.