I haven’t watched any of Star Trek Discovery, so I’m walking in blind and maybe this was already covered or contradicted by that show or others. I’m just commenting based on the post.
If one of the founding member races of the Federation could just leave, it had to have been over some sort of major disagreement. Perhaps something about the nature of the Federation had shifted to such a major degree that Vulcans no longer felt comfortable participating, maybe a turn towards militarism? Or maybe they saw constant disorder within the Federation and thought that their resources were being wasted helping emotional younger races that would never learn.
Maybe there was some sort of “Vulcan First” movement that emphasized the superiority of Vulcans over all others, even Romulans by extension were deemed better allies than the Federation. Or maybe the break from the Federation was orchestrated as part of a covert Romulan psychological warfare operation to try to subvert Vulcan society, a sort of parallel to our modern day Brexit drama and them leaving the EU. Maybe the two efforts (Leaving Federation and accepting Romulan refugees) went hand-in-hand and the break from the Federation occurred at relatively the same time as accepting Romulans back on Vulcan.
I think the Romulans would have less to lose over an invasion of their privacy, rather they’d see themselves as infiltrating Vulcan society as part of a grander goal of eventually assuming power over Ni’Var and subjugating the Vulcans.