Which is a problem with this story too. The original stories all talked about a 20rd magazine. I dont believe there is such a creature as a 20rd Saiga 7.62x39 magazine, just 10 and 30rd, meaning either he reloaded a fresh mag or the 20rd mag aspect of the story is false. I saw the previous post suggesting that it takes some work to modify and AK mag, various sites on the web say its "an easy mod". Either way, while Im not claiming to be an AK expert, I dont believe there is a 20rd AK mag to be modified. This brings us back to questioning the whole mag issue...
I think you're correct about the Saiga mags. Nothing on the exporter's site in that caliber/capacity combination.
Hungary did make 20 rd AK mags for thier AMD-65 version of the AK used by tank crews. These are available just about anywhere that sells milsurp stuff usually for about $5-$8 apiece. Still need to be modified to fit the Saiga.
The problem that has popped up with these DIY modifications is that they tend to cause feeding problems and the rifle jams. Of course if done by a competent gunsmith or if Vang is reasonably skilled it could have been done. Question is: why bother.
IMHO I tend to think he just had a spare 10 rd ready to go. The court papers don't mention the mag though they do give the serial number of the rifle and mention he did not have ammo on him when arrested.