Of course this data loss would also have to include any paperwork and crew memory... So that at no point does anyone seem to remember to report;
"Y'know those Romulans... Well they sort-of look like Vulcans..."
So not quite that easy.
My suggestion would be that rather than a Voyager style, they simply patrol/explore the border. They could still be the spark that caused the war. But your story arc may only feature the occasional incursion/conflict with the Romulans during which time they only ever encounter Romulan ships that either escape or destruct (if your crew win hands down)... Or maybe the occasional Romulan successfully posing as a Vulcan trader, or surgically altered...
That way they still wouldnt know what the Romulans look like, and your story-arc idea remains intact.
Perhaps they are supporting a frontier colony (even more B5-like) and it is the colony that sparks the war, and your ship/crews job to defend them...
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)