A scenario that almost makes sense, as presented here a couple of times already:
The fleet of "peace envoys" was there to make noise that would mask the presence of dozens of not hundreds of cloaked warships. Those warships could never make it through the closely monitored NZ on their own, since the tiniest emission where no emissions should be would give them away. But if they rode in the wake of ships that had a legitimate reason to cross the Zone, then they could get through, and could indeed occupy Vulcan with a sufficient, realistic number of troops and amount of firepower.
Naturally, old Vulcan tubs would make more noise than other vessels - and old Vulcan tubs is something that the Romulans might indeed have in their possession, either dating back to the time of the exodus (Vulcan ships are known to have remained in service for centuries, so why not millennia?), or to recent raids on Vulcan shipping. That they'd have to steal some from the Federation would be akin to the plot of "Moonraker": they quickly needed one more, and thought "what the hell, let's take one from the Feds".
However, there could be a far more sinister motivation behind the choice of ships - one that would explain much more. Perhaps the peace envoy was genuine? Perhaps Romulans stole Vulcan ships in order to lure genuine Romulan peaceniks into them, and then leaked the news to Starfleet which obediently exposed the "invasion plan" (with significant help from Spock who was given the grand tour on the plan and then allowed to escape, but also from a series of hired goons who would steal the T'Pau
in as loud and blatant a manner as possible). As the result, the peaceniks were massacred right in front of the eyes of Starfleet - and the entire galaxy cheered!
It would be a win-win plan for the Romulans: if Starfleet reacted, the peaceniks would be killed, and all UFP enemies would hail the audacity of the Romulan attempt. If Starfleet didn't react, Vulcan would be conquered and held for a few days or weeks, then a retreat would be effected, and the peaceniks would be killed anyway. Political victory would go to Romulus in either case.