Is it really true that "Jump threading, though, can- not be implemented in standard C."? Surely, each computed goto:

  goto *labels[*vpc++];

can be replaced with a full copy of the switch:

  switch (*vpc++) { case OP_ADD: goto ADD; case OP_SUB: goto SUB; }

..and hopefully the compiler will replace the entries for the cases in the switch's jump table with the goto targets.