The canonical Rigellians mentioned in "Journey to Babel" were only said to have blood chemistry similar to that of Vulcans; I've always found it a little odd that people jumped to the conclusion that they were actually Vulcanoid. But they're portrayed inconsistently. Catalyst of Sorrows (which established that they were native to Rigel IV and had a expatriate population on V) says they have a Vulcanoid internal anatomy but look indistinguishable from human (presumably on the assumption that Hengist from "Wolf in the Fold" was a native); but other books say they look just like Vulcans. Cold Equations recently mentioned that Safranski looked like a Vulcan, which I don't recall having previously been established.
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. Especially since there were sufficient differences between recently-separated Romulans and Vulcans to complicate Crusher's treatment of a Romulan downed on Galorndon Core a century later, it's not unreasonable to speculate that Rigelians are as roughly closely related to Vulcans as Romulans.
When did this divergence take place? It may be that the Rigelians descend from an Arretian-era population. Looking at Memory Beta's page
, it seems that the consensus has been to write the Rigelians as a Vulcanoid population that emigrated from 40 Eridani at roughly the same time as the Rigelians. At least one source suggests that the Vulcanoid emigrants settled on the homeworld of the Kaylars, reducing them to a marginalized population on the fringes of the new Rigelian civilization.