The idea of the Voluntary Aid Detachments came about because during the South African War (Boer War), the St. John’s Ambulance Brigade supplied 2,000 men and orderlies to care for the sick and wounded. In 1905 The British Red Cross Society was founded with the aim of supplying help for home defence during wartime. The organisation received its Royal Charter in 1908. With the consent of the War Office, the Red Cross set up Voluntary Aid Detachments as a supplement to the Territorial Medical Service in 1909/1910. A great many Voluntary Aid Societies already existed in Britain at that time, but they all acted independently. At first the Order of St. John in Jerusalem Ambulance Brigade and the British Red Cross were the main organisations supplying volunteers, but Territorial Force Association V.A. Detachments already existed, so it was decided that these Detachments should be co-ordinated and that the County system, which had been followed by the Territorial Force Association, should be adopted.
V.A. Detachments were composed of groups of men and women volunteers and performed many different roles, not solely nursing. These Voluntary Aid Detachments formed part of the technical reserve and their duties included transport, the organisation of rest stations and auxiliary hospitals, as well as nursing.Source: http://www.redcross.org.uk/About-us/Who-we-are/History-and-origin/First-World-War/Card?fname=Mercedes&sname=Tiber&id=208651&first=true
The Red Cross Archive is wonderful for researching VADs in WW1 and most of their records are now available on line. There is also a facility for posting photographs of unidentified VADs.