Iyi vallah means ( if translated literally) I swear to Allah (God) it is good. But generally means it is going good or if said as Iyiyim vallah means I am good really. The typical situation that you can hear it could be like that:
-Merhaba, Nasılsın? Nasıl gidiyor? (Hi, how are you, how is it going?)
+İyi vallah ( it is going good)