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.
The majority of Slavs in that area were perceived as Bulgarians. Following the annexation of Vardar Macedonia by Serbia, Macedonian Slavs faced an effort to adopt Serbian customs and practices.