If the origin wasn't in mainland China, I'd say go for it. The thing is, there's a 10% chance that PEK-HKG will arrive after HKG-Dad departs, and an additional 30% chance that it will get there within 30 minutes of departure. If you happen to catch a semi bad weather day, 9+ hour delays are not uncommon. Flying out an hour or two earlier won't help if you encounter such.

If I were you, I'd either plan on spending a night in HK, or go with the VN option. There are two reasons I like the VN option:

1. The PEK-NNG flight path is MUCH less congested than PEK-HKG
2. There are more flights HAN - Dad than HKG-DAD

As for KA v VN business class, imo it's a wash these days.