The MOS 6561-101 is the later revision of the VIC chip which gives the VIC-20 its name. These chips are new old stock, from right at the end of the VIC chip's production run, and are of course fully tested.
This VIC chip is for the PAL systems only. See 6560-101 for NTSC systems.

Common symptoms of the VIC-20 suffering from a broken VIC chip are:

Blank screen with no sync (monitor usually says "NO PICTURE")

Strange colours on boot-up

Incorrect start-up text for BASIC