The 1931 Polish census has 60% being Polish with another 30% being Jewish. Less than 10% were Lithuanian. Vilnius was the capital of Lithuania for 600 years at that point. Why did Poles so greatly outnumber the Lithuanians in their own capital?
Because there was the Polish-Lithuanian Commonwealth, which was established in 1569 and lasted (nominally) to 1795. This Union essentially joined not only those two aforementioned kingdoms; but it encompassed Białoruś (Belarus), Ukrainę (Ukraine), and so on. As a result, Polish became the most-spoken language within the Commonwealth; thus, over time, people merely stuck with what worked for them.