Changes
/* Third schematic Z80+ROM+RAM+VGA */
So you put the two last schematics together and tadam... got a problem.
The problem is that two components are accessing RAM in at the same time: the Z80 and the VGA, so you had to make a sequencer. A sequencer is simply a counter fed by a clock: 00, 01, 10, 11. And you manage work task like this:
*00 RAM WRITE start from Z80
