As has been said, the Vas-R2 is an ntec reskin, and the C9 is an OCA reskin
You can get 0 and 1 slot ATACs from the joker store (Yes, not the 3 slots of ARMAS)
All legendaries can be bought in game (Ive owned them all, never bought a JMB)
But for what its worth I dont even find those guns to be great (I own both and use neither)
Ursus is same ttk, same range as Ntec, just 1 less stk but with a slower rate of fire, this also makes the Ursus slightly less forgiving in cqc (Ursus does have a slightly further min damage dropoff as well)
The Yukon is the fastest ttk in APB at .45 seconds, but is only effective in that mode up to around 7m. In its marksman firing mode the Yukon is a slower RFP (.45 burst interval vs .35 for the RFP).