I remember my parents telling me that at one time the corner lots were zoned for apartments or flats (as we called them. This could explain why some lots were left vacant. I also remember interior lots being vacant long after the rest of the block had been built out. We lived on 22nd Ave. between Judah and Kirkham, where most of the houses had been built in the 1920s. There was a vacant lot on the westerly side of 22nd that was not built on until 1948.
There were also vacant lots on the north side of Judah between 20th and 21st avenues, and on the south side of Judah between 18th and 19th avenues.
These vacant lots were great places to play.