No, Vulcans and Romulans are identical.. the both originated on the planet Vulcan...
the only difference between the 2 races is that back in the dark ages of the Vulcan history (as explained in TOS by Spock) Vulcans were overwhelmed by their emmotions making the Gengis (sp?) Khan clan look like a bunch of sissies. One Vulcan stepped forward and started the philossphy of Logic, and from their it turned into their religion, then into their culture, then into who they are as a being..
now, as pointed out in TNG, back in the ancient days of Vulcan.. the Vulcan race split into 2 factions.. one's who embraced thier emmotions and shaking logic as a lifestyle (though still use logic to make decisions) called themselves Romulans (where name came from is unknown) and the 2 factions of Vulcan civilization fought each other.. Romulans to destroy the Logical sect that had been established, Vulcans because it was logical to preserve their way of life.. thus the war raged.. in the mean time technonogy progressed and space travel progressed then deep space, then Warp for Vulcans.. at such time, the Romulan clan were on the brink of losing to the Vulcan clan since Vulcan's were using flawless logic to plan and wage war... such were the Vulcan "Dark Times" as the shows call them.. the Romulans on the brink of defeat set off into deep space not to be heard from again for many generations / centuries ???? then Vulcan's discovered Earth and Humans were capable of Warp Travel and thus first contact.. and let the rest of TOS history write it in...
as such, there is no difference between the looks of a Rom and a Vulcan what so ever.. they are the same race.. however since time has past so much between worlds, the Vulcan's call them their Cusins...
Why do you think that Spock was repeatedly trying for reunification??? to bring the Romulans and Vulcans back together as 1 race instead of 2....
This was the Vulcan Civil War...
think of it as the North vs the South..
Uninted States of America vs Confederate States of America
Yankees vs Rebels
same difference.. all because of different points of view...
that is why after watching several episodes of enterprise.. T' Pol fails a lot at holding her emmotions in.. only a Vulcan very lax in logic (which a Vulcan starts to learn from the day they are born {cirta Undiscovered Country, Spock remembers his birth}) would show this level of emmotion that T' Pol shows.. and before they are even considered TRUELY Vulcan, they go through several rituals (Cirta : TMP), usually done at an early age... around 20 or 30.. something like that (Vulcans age much slower than Terrans, usually living 200+ years (Cirta TOS, ST: IV : TVH, TNG x2 episodes).. so Logic would be foremost and as preached in Tos, TMP, and in episodes of TNG and on Voyager.. Vulcans showing emmotion is a disgrace, a slap in the face of Logic...
So, with as much emmotion as T 'pol shows in Enterprise, one can only conclude that she is actually Romulan acting like a Vulcan, thus slipping up ocassionally by showing blatant emmotion.. The reason Archer and crew know no difference is #1 humans dealt with Vulcans rarely (Cirta Enterprise Episode 1), and #2 Supposedly no Human had has dealings with Romulans.. or if they had, they hadn't lived to tell the tale...
that is my reasoning behind T' Pol... Hope it makes sence...