Going by this transcript
of "Journey to Babel", Spock's line is "Rigelian physiology is very similar to Vulcan". It's not just a matter of blood chemistry, but the entire Rigelian organism is similar to the Vulcan.
In DS9, Inter Arma Enim Silent Leges,
Tuvan's Syndrome was said to mostly affect Vulcans, Romulans and Rigelians, so apparently their neurology is essentially Vulcanoid too. So it seems the writers of that episode interpreted the line in JtB as Rigelians (this race of them, anyway) being similar to Vulcans in more than just blood composition.
As for where these Rigelians truly came from, I like to assume Arret is the answer - and the "looks-Human" VS "looks-Vulcan" issue can perhaps be explained as different groups descended from the same biracial population, given that Arretians have been confirmed as two species, one Human-like, one Vulcan-like. But I guess colonists from Vulcan could have settled there too - Catalyst of Sorrows
does mention a period of colonization post-Sundering, when Vulcans established themselves on a string of planets along the later Neutral Zone (e.g. Sliwon). So offshoot populations across the Beta Quadrant besides Romulans apparently exist.
Face it, Spock, it's hopeless. The genie's out of the bottle