I guess maybe they are trying to dump an object which writes vector instructions by .insn directives, so the elf architecture attribute or mapping symbols won't have v.  I will suggest two solutions here rather than just modify the default_arch,

1. using -Mmax option

2. Allow to set the default arch from the configure option, like what gas did before.  Not sure if it is possible to find a way to share the configure set both for gas and opcode (assembler and dis-assembler).

BTW, force setting default_arch to rv64gc or whatever in dis-assembler isn't really good, I think that I just forgot to update it before.  So I sent a patch to let riscv_parse_subset unify the default architecture both for assembler and dis-assembler from the riscv_all_supported_ext table.

Nelson