I got the VecMulti sd cart from http://www.vectrex.biz... would not work. Booted, but gave me nothing but garbage on the game select screen, no matter how many times I tried wiping the SD card, reseating the cartridge or the card, nothing. Had some emails back and forth with Richard, he said that once before he had someone report the same problem, but his OTHER flash cart for it (the one that you flashed with USB rather than using an SD card) worked for that guy. So I exchanged them, and sure enough it works. The game select screen is still very dim, which is weird, but it works.
WHY would the one cart give a problem for only 2 random Vectrexes out of all the ones he's sold? No idea. But it does, because he verified that the cart worked on his.
So yeah, there's my story. The good news is at least I have SOME kind of working flash cart for it
Also, oh god that audio buzz. I really need to get the mod done to reduce that.