I've never flown VN, but my boss did 3 months ago, and told me he was quite impressed (a350).
If the fare is reasonable, I would book VN, simply because no other airlines have as many flights to DAD (well, actually not using the PEK-HKG flight path is also a BIG carrot).
What many people don't understand about PEK-HKG, is that tons of flights (not just China) clog it up. In fact, PEK-SGN uses it, but PEK-HAN does not.