I can't really see Vergil using guns even in his earlier childhood. But he obviously knows how to use one(even a 5 year old is smart enough to know how to use a gun.). Also the whole jackpot scene is traditional and I doubt it had anything to do with them shooting guns together in a earlier time, it was most likely a childhood phrase they did or made up just for fun. Think about it, kids using guns with their mother around(must be bad parenting)? Any way Nero says jackpot in DMC4 when he busters Sanctus now if it was a gun thing, you would think he would've lv3 charge shot him or something. Btw the jackpot in DMC4 was lame.