Alright, success! This 12+ year old thread finally reaches a happy conclusion. I am very grateful for all the invaluable input you guys have given, my very first computer now comes back to life after a quarter of a century in oblivion. Thanks!!
The problem turned out to be:
1. The broken fuse in the PSU.
2. A broken trace in the VRAM network. Specifically, IC12 didn't have a connection between pin 2 (DATA IN) and the ADDRESS7 pins on the VRAMs/VDP. Soldering in a jumper from pin 2 on IC12 to pin 13 on another VRAM solved this. The broken trace probably stems from the process of desoldering the old VRAMs and soldering in the new sockets, some force had to be used here and there...
Checking the service manual was crucial:
Quick and dirty fix:
It works, cheers everybody!
This thread is an awesome read. Good to hear you managed to resurrect your MSX! 
Wow, what I nice thread to read. Next time check back more often, could have save you replacing the VRAM