This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
When I was first in need of a VIC20 diagnostics cartridge, I made one out of a faulty Super Extender cartridge.
VIC20 Penultimate Cartridge boards. These also ran a selection of cartridge games and worked as a memory expansion.
2022 Update: The Penultimate Cartridge includes built in VIC20 Dead Test+ and is now available from The Future Was 8 bit