hi Tuesday,
have gone the Hanoi to HK route a couple of times. No problems anytime, easy and straightforward.
You can get the visa for China in HK, no prob, 1/2 days max. Just be sure to have passport photos with you for all visas.
When travelling on the bus or train in China and Vietnam watch your bags CLOSELY, as we almost got nabbed. Be sure to have a Lonely PLanet with you and Berlitz phrase books are handy and small.
No problem in getting from Phonh Penh to Bangkok, by bus all the way and not that far in time. Friends of ours have just done it a week mor so ago. You can also hire a mini van (merceedes) if you want more comfort, and not to expensive if a group of you sharing.
Go for it, enjoy it, the people, the food. Wonderful. We also have done the Bangkok to KL route, but not as far as Singapore.
Optimistic