IDSDM is a dual-SD module to boot (visible from the BIOS as a boot media) an ESXi (Windows or Linux is not supported).
It is unrelated to the vFlash (also SD card format) used by the idrac (except for specific blade server where the slot is shared, i.e. it can be used either as a IDSDM or as vflash).
The vFash is a storage card that used for storing configuration files, ISOs, or any other file used to install/configure/diagnose the server from the idrac. vflash may also be used to boot an OS (typically an OS install ISO) but it is not redondant (single media as opposed to IDSDM) thus is not recommended as a general purpose OS storage.
vFlash does require an iDRAC enterprise licence (can be purchase separately).