The vDrive is an external Microdrive hardware emulator for the QL, or the Spectrum with Interface 1 expansion. It simply connects to, and is powered from, the existing Microdrive bus. Due to differing connection requirements, there are two versions of vDrive.
To complement the vDriveQL there is also the vMapQL which is a configurable hardware device for translating Microdrive addresses. See the vMap page for more info.
A quick feature summary of all devices can be found here.