The Vectrex core is '31kHz only', I wrote about it's sync problems formerly in the core's own topic:viewtopic.php?f=115&t=33150
Found the reason of incompatibility with my TV: the core provides a composite (horizontal+verztical) 31kHz sync signal over VGA pin 13 instead of pure HSync. Discovered it accidentally, forgot to switch on my sync merger on my DIY RGB->YUV converter, and despite it the result was an acceptable picture (see screenshots). Probably it won't cause any visible difference on monitors being less picky than my TV.
Another issue may be the polarity of the sync signals: the standard says both of them should be negative at 640x480@60Hz, but i.e. in case of Apple ][ core both H and V syncs are positive. At 800x600 resolution both should be positive, but one is negative in case of GameBoy core.
See VGA specifications here:http://www.javiervalcarce.eu/html/vga-s ... cs-en.html
You may use logical gates (members of the SN74 IC family) to reverse sync polarity, or merge them to C-Sync, but it would be more elegant solution to handle these problems on core level (but I'm not familiar with VHDL/Verilog).
More info about it in another topic of this forum:viewtopic.php?f=115&t=31322&p=339730#p339696