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.
It’s curious that despite this cultural revolution that led to founding of planet Vulcan they are still basically equals technologically. It doesn’t appear that a few thousand years of excess emotion and violent tendencies has been at all detrimental to Romulan technological advances.
Their physiology is barely diverged so their intellects are likely to remain similar. Espionage is frequently the theme of Romulan encounters, which would help keep them up to date. And if they procreate more frequently than every seven years, they might have a much larger population even with greater murderousness, with more people being advantageous for tech development.
Gene Roddenberry envisaged the Romulans as Star Trek’s version of Communist China and the Klingons as its Soviet Union. In Making of Star Trek, he and Stephen Whitfield describe the Romulans as “highly militaristic, aggressive by nature, ruthless in warfare.”
That’s from the man himself. Not sure where you got the impression that romulans aren’t violent.
It’s more like how the Borg are described as an unstoppable unrelenting all powerful force… and are stopped, relent, and are devoid of power. On paper they are one thing, on screen they are another.
With the Romulans, they tend to outsource the violence. Pit party A against B, then clean up after. Practically scavengers. Klingons, Jem’Hadar, and Hirogen I’d more readily describe as violent.
You bring up a good point. It feels like Romulans have also learned to control their violent emotions, but rather than suppress them entirely that energy is just focused all into tactics and smarter ways to be ruthless.
When I think of the Romulans, I first think of that time they plotted to bomb DS9 to gain control over the wormhole only to be thwarted by time-traveling O’Brien