That seems rather pointless. If you're going to go through all that, Vietnam/Indochina might as well have been thought of as Outer Mongolia was- somewhere China can absorb into the empire.
It really was pointless, that's why China bothered trying to make Vietnam apart of its empire. There's a chance northern Vietnam could've been readily absorbed into a rapidly growing Ming say (it almost did) or had the Mongol Invasions succeeded for sure there's a great chance it could've been a Chinese population there. There's more then enough people in China to displace those in Vietnam over a millenia but unfortunately the manpower, the money and the dedication to passifying a jungle way out of the way of civilization I mean.
It's not like it could never have been a province it just, there were alot of people in Vietnam- part of why their southern colonization was so damn successful (there were so many!).
I'd say the farthest reaches of the Qing were REALLY big. I mean, parts of northern Vietnam (Red River Delta and north), maaaybe bits of northern Korea (areas north of the Yalu river are already pushing it and even now are very heavily Koreanized, albeit lots refugees from the Korean War) would be like, breaking point for China. It ballooned around the conquests of the Qing.
You gotta remember there has to be some economic and political incentive as with all colonization as well. I'm sorry I know I'm putting up more barriers then I should, I'm not helping much.
What I think perhaps, a successful Mongolian Invasion of Vietnam would definately give a chance to be a province of China, and parts across the Yalu River could've been absorbed if you could find a period in Korean history when it was fractured kingdoms.