I think it would be perfectly in character for the Romulans to think that while Vadic's changelings focus on destroying the Federation (and we might be taking about something that Oh had been aware of for decades), they figure out how to neutralize changelings so that by the time they come for them, they'll be prepared. And the demise of the Federation is the endgame for the Romulans too anyway, Vadic is doing them a favor by doing the dirty work for them. They might think all they need to do is avoid a repeat of the Lovok fiasco and they could come swooping in, taking all the scraps for themselves and establishing themselves as the premier political power in the Alpha Quadrant.