What a first taste at backpacking! I hope you love South East Asia as much as I did, Laos being on top of my list.
Starting with Hong Kong, then Vietnam will probably prevent you from experiencing a deep culture shock. Good itineray overall. Do you have a more precise idea of cities you want to go to? Budget wise, are you ready?
Just a thought for S.E.A. : 2 months for 4 countries might be a bit tight, because of the slow pace of life that will get to you wether you want or not. You should try to keep your itinerary as flexible as possible, so that you can extend your stay when you fall for a particular place (of course within the period of time allowed by visas), or the opposite, leave early an uninteresting city.
And, welcome on the boards !