In RINIT:
    zend_function *orig;

    XG(var_dump_overloaded) = 0;
    if (XG(overload_var_dump)) {
        zend_hash_find(EG(function_table), "var_dump", 9, (void **)&orig);
        XG(orig_var_dump_func) = orig->internal_function.handler;
        orig->internal_function.handler = zif_xdebug_var_dump;
        XG(var_dump_overloaded) = 1;
    }
In ZEND_MODULE_POST_ZEND_DEACTIVATE_D:
    zend_function *orig;

    if (XG(var_dump_overloaded)) {
        zend_hash_find(EG(function_table), "var_dump", 9, (void **)&orig);
        orig->internal_function.handler = XG(orig_var_dump_func);
    }
And xdebug_var_dump() is just defined as a normal function.