CRTCs 1/2 have a fixed VSYNC width value of 16. VSYNC width can be configured with R3 on CRTCs 0/3/4. If 0 is programmed this gives 16 lines of VSYNC.
The 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. On all CRTCs, the comparison between VCC and R7 is disabled during a VSYNC. So, even if R7 is changed, a new VSYNC cannot be triggered during a VSYNC.
The HSYNC and VSYNC signals from the CRTC are passed to the [[Gate Array]] for further modification. See its wiki page.