It would stand to reason that Vulcanoids on both sides of the border would actually think warmly of unity - if just the other side would give up their despicable way of life! It seems Spock chose the more difficult approach out of the two possibilities: convincing logical Vulcans to at least consider a Romulan way of life would be easier than convincing emotional Romulans to consider anything at all
, including the Vulcan way of life.
Also, unity in which Vulcan would join the Romulan Star Empire appears more easily achievable than unity in which Romulus joins the United Federation of Planets. Perhaps Spock avoided the easier approach because he wanted to steer well clear of the very tangible threat of Vulcan joining the RSE?
He's not trying to convert them
...But how is that consistent with this statement?
"The answer has been here before us all along. An inexorable evolution toward a Vulcan philosophy has already begun. Like the first Vulcans, these people are struggling toward a new enlightenment."
It does seem as if Spock believes that conversion to "Vulcanism", perhaps even Surakism, is the only way to unity. If he's not going to promote that, then what is he doing?