Maybe someone can write a story about this.
Plus I'm sure Vreenak's many enemies welcomed his demise, and didn't particularly care how it was achieved.
Probably true. But Vreenak probably also had his share of allies as well, within the Tal'Shiar and the military, I would imagine.
Even so, I can't see the Romulans of all species allowing that precedent to be set.
I agree. I don't think the Romulans would want the Federation to get into the habit of assassinating their people.
If the Romulans ever did find out that Vreenak was assassinated, there would probably be some factions of Romulans that would prefer to let bygones be bygones, since things turned out well in the big picture of things. But there would undoubtedly be others that would want payback.
I can imagine Vreenak's allies in the Tal'Shiar plotting revenge as soon as they discover the sordid facts of the matter. A discriminate and proportionate response would be par for the course. That would mean taking out Garak, preferably with an explosion; and possibly taking out Sisko too.
But since Sisko was no longer around, maybe they would target Bashir instead, with a toxic biomimetic gel, because they might have assumed that Bashir was involved in the scheme when Bashir signed off on supplying Sisko with the gel that Sisko needed to trade for the rods. An eye for an eye, etc. At that point, they would theoretically be even.
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Here's another "what if" angle to the story: what if Sisko's log entry, where he admitted to everything, wasn't actually successfully erased.
What if say, O'Brien, or whomever, while doing routine maintenance on the station's computer, uncovered Sisko's log entry intact, or at least the gist of it intact; and then the log entry wound up being leaked out to the public.
How would Kira, Worf, Odo, Dax, and especially Jake and Bashir react? Bashir might be the most righteous of the bunch. The info might not sit well with Bashir. What would happen to Sisko's reputation?
ETA: The writers were clever not to have any of the other characters, besides Sisko and Garak, in on the plot. Garak is a natural for this type of operation. By not including any of the other characters in the scheme, the writers avoided the potential of unnecessarily sullying any of the other character's reputation with the audience, that is, assuming the audience would view the plot/assassination negatively.
On the other hand, it can be seen as a cop out by the writers. Since none of the other characters knew of, or found out, about the plot/assassination, Sisko never had to answer for his actions to his colleagues; unlike how Bashir confronted Admiral Ross in "Inter Arma Enim Silent Leges", and thus forcing Ross to answer for his actions in that episode.
I wish Bashir would have found out about this. I can only imagine how Bashir would have reacted and what he might have said to Sisko. Of course, if the writers had done that, they would have only made things more complicated for themselves.