I was thinking of moving and in that case wanted to replace my ST carrier with something a bit more local. I am still working out the minor details, like where. But I will end up either in Japan, China or Vietnam, in that order.

My flights will be within those countries as well as to and from those countries. ( Tokyo - Osaka - Beijing - Hong kong - Hanoi- Ho Chi Minh )

China Southern, China Eastern, Korean Air or Vietnam Airlines?
CZ has 121 destinations on 340 aircrafts. MU has 104 destinations on 263 aircrafts. KE has 116 destinations on 132 aircrafts. VN only has 46 destinations on 66 aircrafts.

Korean air seem to have a relationship with US Bank/ Credit cards.
China Southern lets you earn miles on China Airlines, until they do join ST.

Which SkyTeam carrier has the best Asian FFP?

For the list you've made, I would choose China Southern and Vietnam Airlines. I've never flown them, but I imagine that the service would be quite good. I've heard only good things about them both.

good luck mate. i wouldn't touch Vietnam airline or Chinese airline with 10 feet pole if i had a choice, but hey! it's up to you.

Do you care to broaden that statement a bit? Is there anyone that offers lifetime status of the Asian carriers?

Once you credit 500,000 miles on KE or other SkyTeam carriers to their program, you earn Morning Calm Premium Club, which gives lifetime SkyTeam Elite status.

Well, I guess here is just about personal choices. Indian, Korean, Thai, Malaysian, you name it, all Asian carriers seem to be good, so Chinese I would imagine, would be the same.

CZ is actually a very good airline. Not sure about VN, but I've heard some stories here and there.

As for FFPs, do note that all Asian SkyTeam FFPs (VN, CZ, KE and soon GA, CI and MU) have mileage expiry dates attached to their miles: KE miles expire after five years, CZ, CI and GA miles expire after 36 months, and VN miles expire after 24 months. No information on MU.

If you're aiming for a fast way to earn elite status, GA gives you Silver status after 10,000 qualifying miles, and Gold status for 30,000 miles. If you're in a higher tier and you don't reach the number of miles needed to stay in your tier (for example, Gold), then you will be downgraded to the tier immediately below. So non-qualifying Golds become Silvers, Platinums become Golds, etc.