Maybe he met an Italian named like that? It's really hard to come up with if you're searching for random Italian surnames. As far as I know there are not famous Italian named Vargas apart maybe for Daniele Vargas an actor whose real name was Daniele Pitani. However it's not that famous and died in 1982 so, unless Himaruya is a fan of old Italian humoristic film I fear he didn't take it from there.
Some suggested it was from Alberto Vargas, a pin up artist, but the guy is Peruvian... In the movie Thunderball, there's also a Vargas working for Emilio Largo, an Italian enemy of James Bond. I've no idea if said Vargas is also Italian in the movie/book (I'm not a fan of 007) but since his boss is Italian Himaruya might have though he was too.
Anyway, I really have no idea why he picked this surname up.
As I've said Italy is definitely not satisfied of how Veneziano and Romano had been called...