I don't have the numbers right now, but VDC mode for the intro picture doesn't have the same total number of vertical lines as a PAL image. I tested it on an old CRT TV and a modern(ish) LCD and both were able to lock to the vertical sync.
Since you have the proper hardware, would you be willing to run some tests with various timings for me?
Sure, no problem, happy to test.
IMO the exact number of lines itself shouldn't matter that much, a CRT will simply obey the vertical retrace signal when it gets it. However, changing the number of lines also means either you change the fps (which should be 50Hz) or the horizontal frequency (which should be 15625KHz). Since the 1084S accepts anything between 50-60Hz, I wouldn't expect you get issues with vertical frequency that quickly, the horizontal frequency is likely the most critical.
