They were the same species on the same planet just a few thousand years ago, which is the blink of an eye in evolutionary terms.
It might be more accurate to say that Vulcans are just Romulans with impulse control. Before the split, Vulcans were more like modern Romulans than modern Vulcans. Vulcans at that time were highly emotional and violent. Then they had a sort of cultural revolution, which involved controlling emotions and focusing on logic. This led to some traditionalists leaving Vulcan and founding Romulus, carrying that emotional and violent culture with them.