The problem I have with the notion that Garak's "real plan" was to kill Vreenak is that it makes him out to be particularly linear. Indeed, it makes him out to be an incredibly inflexible spy, when he ought to pursue multiple avenues and objetives at any given time. His lying--and his practicing at lying--reveals someone who is always working different angles of the same problem. Perhaps I can go further: playing complicated games that unfold over the long term seems to be a unique trait of the Cardassian political class, and we should be no more surprised that Garak might both fool Vreenak and kill him than Dukat might both help Sisko and pursue an alliance with the Dominion. They keep several balls up in the air.
I don't think linear is entirely bad.
In chess, for example -- at least at higher levels of play -- there is a certain progression that follows an opening move. This is why masters can claim, with confidence, checkmate in X moves. The chain of events is prescribed, and that they can see what others cannot it is what makes them masters. I think Garak is such a person.
Maybe he didn't know if the rod would pass inspection or not.
Maybe he didn't care. Maybe Tolar and Vreenak were pawns that were always destined for sacrifice in setting up a game board that would, ultimately, manipulate the Romulan government into submission. Consider that ordinance Garak delivered wasn't the data rod at all: it was a warhead filled with mystery and fear.
Arguably, if the rod
had passed muster on Vreenak's shuttle, and Garak allowed Vreenak to live, it might have been
less of a sure thing because there would be even more advanced analysis available on Romulus -- thereby increasing the risk of detection. Moreoever, there would be no mystery for the paranoid Romulans to solve, and no fear over what they don't know. And then they'd have Vreenak's impressions/suspicions of Sisko, and their general distrust of the Federation as a source of the data. Even if the forgery was perfect, they'd have good reason to doubt, and no reason to act hastily.
In order to really get the Romulans' attention, Vreenak had to go. And I think Garak knew that from the very beginning. It was just a matter of setting up the circumstances of his death, and the right point at which the pawn should fall.