After my earlier complaint regarding the lack of response from Vortex, Dave Morgan from NewPro UK (UK Distributers of Vortex scopes) has kindly responded and provided the information I was looking for. (Thanks Dave).
Below is a summary of the information I received :
The Apochromatic lens system using the 2nd element of HD glass does account for most of the price difference (between the Razor and the Viper) and the zoom does give an extra 0.1 degree in the field of view throughout the zoom range.
Although the brochure states that both scopes use 'premium' HD glass, within that 'premium' category there are sub-categories with the Razor using the best there is for both elements and the Viper using a lesser grade of 'premium' HD glass for its single element objective lens.
The internal workings of the Razor are not entirely from HD glass, the objective lens and optical system of the Razor HD is more highly engineered than that on the Viper. The Razor having a triple apochromatic system which has 2 elements of HD glass in it for the almost complete elimination of colour fringing across the whole FOV as opposed to the Viper which only has the objective lens itself made from the HD glass. This, combined with the extra 5mm of HD glass in the 85mm objective lens of the Razor naturally adds to the price compared with the 80mm Viper HD and the zoom lens of the Razor is also more highly engineered than that on offer in the Viper HD, giving a wider FOV throughout the range.
As an added benefit of the Razor, it will also fit the Swarovski lenses so should anyone fancy their 30xSW (my choice of eyepiece on it as it is about 15% wider FOV than the Vortex 30x lens and is edge to edge sharp), 20-60x zoom (although there is very little to choose between this and the stock Vortex 20-60x zoom) or 25-50x extra-wide zoom then you can use these instead of the Vortex eyepieces.
In addition to the information I received I also found this review on the web :