This is an old post, preserved for reference.
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I have been working on various VIC20 cartridges recently. One that I was interested in building was an IEEE-488 cartridge. I had previously tried out the PET microSD with a VIC20 IEEE-488 cartridge, and found it worked well.
2022 Update: I gave up with the IEEE-488 cartridge in the end, the requirement for a ROM means it is always going to take up space that might be needed for the programs you want to load, so best just stick with an SD2IEC for the moment.