Here is a re-cap of the process.
How to obtain your VID?
An individual can obtain his / her VID from the UIDAI website:
The individual enters his / her Aadhaar number and obtains and OTP on registered mobile. After entering the OTP, a VID is communicated to the individual.
A VID is also generated by downloading an eAadhaar document. (It appears under the Aadhaar number on the eAadhaar document)
How to use a VID for eSignatures?
During the eSign process the individual is directed to the ESP gateway, and the VID has to be entered on the ESP page. An OTP is received (as earlier) and the eSign process goes forward.