Per Britannica itself the bulk of the Slavs there were regarded as "Bulgarians". Immediately after annexation of Vardar Macedonia to the Kingdom of Serbia, the Macedonian Slavs were faced with the policy of forced serbianisation.
According to Britannica, the majority of Slavic people in that region were considered Bulgarian. After Vardar Macedonia's incorporation into Serbia, Macedonian Slavs encountered a policy aimed at promoting Serbian culture among them.