I know the difference between paint and primer, thanks, however:
http://www.dakkadakka.com/dakkaforum/posts/quote/0/1193074.page
I had a thought though - what if "white primer" is just the name of that colour and it isn't a primer at all. I promptly sent an email to the friendly guys over at Vallejo and they told me that the "white primer" is in fact the name of a colour, and it's properties are no different to any other paint in the game colour range.
I was surprised but not dissapointed at all - as they responded super fast and provided me with friendly and helpful service. Well done Vallejo.
The gentleman I spoke to (his name was Alex) also told me that primers are usually solvent based, but are still just colours, and there would not be any problem using ANY of the vallejo colours as a primer coat.
Also various posts pointed to the "vallejo primer" being a bit harder to work with, but people were ambiguous and didn't properly distinguish between the model color (true)primer and the "paint" from the game color range.