Vulcan is supposedly a worthless rock in terms of resources, and its strategic position at the apparent threshold of Romulan space would not change much for the worse even if it ended up being an enemy world.
I don't know if I can buy Vulcan being right on the edge of Romulan space. Was that stated in the episode?
I understood that it was relatively close earth and Andor (Andoria?) which would seem to put it deep in Federation space. Which would make it a little awkward if the Romulans decided to set up shop there (even with the Vulcans blessing). It would be like Montana giving themselves over to Chinese rule. The rest of the U.S. just wouldn't stand for it.
And, again, there's the issue of crossing the Neutral Zone in order to get to there. Just the act of the Romulans traveling to Vulcan would be an act of war against the Federation.