The problem with that (in C++) would be that

  A a;
  A* p = new A;
  assert(sizeof(*p) == sizeof(a));

This wouldn't be true and cause all sorts of trouble (arrays, etc.) if a had no pointer to a vtable, but *p did. Moreover, you can take the address of a stack/member object, so how would you distinguish the two types? You'd literally need to modify the type system, and that would cause trouble with templates, etc.