On CRTCs 0/1/2, the sole condition to trigger a VSYNC is that VCC=R7. While on CRTCs 3/4, it is necessary to have VCC=R7 and HCC=0 and VLC=0 to trigger a VSYNC.
Also, on CRTCs 3/4 , HSYNC occurs 1µs later than on CRTCs 0/1/2.
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.