These two pulses are related in just the same way that horizontal sync and horizontal drive are related as shown in issue 1. Cameras use vertical drive to develop a ramp to drive an electron beam from top to bottom of the image tube. Video tape recorders strip vertical sync from the input video and compare that to tach pulses from the head scanner and then adjust the speed and phase of the scanner, if necessary. Play that back and the picture rolls for a few moments. Make a test at home with your VCR, by just putting it in Record and then switch channels while it is in record.
Monitors use vertical sync from input video to steer the vertical oscillator.
If your system has all sources synchronized, at least vertically, then vertical interval switching is a subject that might concern you. The reason is so that the VTR or monitor or whatever will have received the entire field of video rather than the top half of the screen from one source and the bottom from the second source for that moment of the switch, not a big deal in the scheme of life, but a bit more visually appealing. Well, all of our vertical interval switchers will accept the command and wait until the vertical interval of the signal currently 'on air', and then switch to the new source, sort of a half of a vertical interval switch, and the best attempt to minimize the resulting trash. The schemes can get a bit more complicated, like switching exactly on line 10, but I'll put those away for a later time. If your system uses editing or production switching, then you have to pay attention to horizontal sync.