zbx_vector_str_destroy() does not release memory used by individual strings contained in the vector. You have to do it explicitly, for example:
....
for (i = 0; i < c_values.values_num; i++)
zbx_free(c_values.values[i]);
zbx_vector_str_destroy(&c_values);
Check Zabbix source code for all places where zbx_vector_str_destroy() is used to avoid memory leaks.