On all CRTCs, a new VSYNC cannot be triggered if VSC!=0. So we cannot trigger a new VSYNC during a VSYNC. The same goes for HSYNC.
On CRTCs 3/4, HSYNC occurs 1µs later than on CRTCs 0/1/2.
The HSYNC and VSYNC signals from the CRTC are not directly connected to the display. They are passed to the [[Gate Array]] for further modification. See its wiki page.