The Apple ][ hardware did not have a direct way to track vertical blanking. There were hacks that used artifacts in the dynamic memory refresh to sniff out when the VBI was happening.
Ah, thank you, that's interesting. I don't recall puzzling through that exact machinery.
But I had a IIe, not a II, and looking at resources for that- yeah, I had a rev B motherboard, and remember well working in the double hi res context, both black and white and color:
http://www.deater.net/weave/vmwprod/megademo/vapor_lock.html