Is Vandex a radon barrier as well as a tanking material suitable for application in a basement?

No. This is because Vandex (http://www.cellars.co.uk/products/vandex-cementitious-waterproofing-products) is vapour-permeable and therefore will not act as a barrier to radon gas. A separate radon barrier will have to be specified as part of the design detail. This could take the form of Oldroyd Xv membrane (http://www.cellars.co.uk/products/cavity-drainage-membrane-oldroyd-xv-clear), double-sealed at all joins in order to comply with regulations. It is best to consult our technical department re: the specific detailing of this.