Are you naming the channel itself in Vietnamese!, if yes then that's the problem.
it's better to write the name in Latin letter (Name), and for Vietnamese you can write it in (Display Name), because the mcms urlrewriter is using the channel name in the url, even if that works sometimes it will cause problems for your users because the url may not render right in some browser.
e.g http://yoursite.com/kho^ngda^'u/