looks like it might be encrypted
aside from the first 0x8000 block which is all 0x00 the rest of the 'empty' space is filled with either
so if you you either of those patterns as a xor mask you convert those blank areas to 0xff or 0x00 which seems more reasonable, although would need to check if there's any bitswap too.
could of course be done like that to throw people off too.
bit weird the first 0x8000 is just 0x00 tho.
although even if you apply such decryptions there are no vectors in the first area of the rom (0x1ffee)
there do appear to be some vectors, starting after that, starting at 0x21ffee and repeating every(?) 0x200000 after that, but that would give a bank size of 0x200000 and mean the initial bank can't be 0.
unfortunately even if you try using a higher bank, the code isn't valid, so there's probably bitswapping, or additional address line fiddling too (assuming it's a good dump)