Article: Throughout their history, Slavs came into contact with non-Slavic groups. In the postulated homeland region (present-day Ukraine), they had contacts with the Iranic Sarmatians and the Germanic Goths. After their subsequent spread, they began assimilating non-Slavic peoples. For example, in the Balkans, there were Paleo-Balkan peoples, such as Romanized and Hellenized (Jireček Line) Illyrians, Thracians and Dacians, as well as Greeks and Celtic Scordisci. Over time, due to the larger number of Slavs, most descendants of the indigenous populations of the Balkans were Slavicized. The Thracians and Illyrians vanished from the population during this period – although the modern Albanian nation claims descent from the Illyrians. Exceptions are Greece, where the lesser numbered Slavs scattered there came to be Hellenized (aided in time by more Greeks returning to Greece in the 9th century and the role of the church and administration) and Romania where Slavic people settled en route for present-day Greece, Republic of Macedonia, Bulgaria and East Thrace whereby the Slavic population had come to assimilate. Bulgars were also assimilated by local Slavs but their ruling status and subsequent land cast the nominal legacy of Bulgarian country and people onto all future generations. The Romance speakers within the fortified Dalmatian cities managed to retain their culture and language for a long time, as Dalmatian Romance was spoken until the high Middle Ages. However, they too were eventually assimilated into the body of Slavs.

Now answer this question: Who vanished from the population of the Balkans?
The Thracians and Illyrians