Maybe the Slavs are considered good slave stock by the West too? Religious objections of enslaving fellow Christians aside (and that's if the colonizers even are Christians) were there any locations where you can have White slave plantations without the climate killing them off en masse?
Maybe if the Slavs had largely avoided Christianization for at least a few more centuries, just long enough for the New World to be discovered and exploited by Western Europe, they would have been considered acceptable targets for slavery by political and religious authorities. The process might still be gradual, though, in the same way that African chattel slavery in the American colonies evolved out of the racially neutral institution of indentured servitude...
Or, to reverse things, maybe the Norse stay pagan and engage in much more effective colonization of Vinland, Markland, and points to the south and west. Once the
skraelings start to die
en masse of disease, the Vikings bring over their thralls from Europe - Christian, heathen and otherwise - to do the hard labor.