The PSU of my VIC-20 died and so I had to find a new one. As my VIC has a two-prong connector this is more hard to find. Searching the internet I came across this forum-thread on Denial. It says that a two-prong PSU is nothing more than a transformer with a fuse. All logic is inside the VIC. It tells you can even use a simple AC/DC adapter with an output of somewhere between 9 and 12V, 3A.
So I cut the two-prong cable from the old PSU and soldered it to the output lead of a 12V 2A AC/DC adapter and the result is perfect.