Well, I don't think research has been done- or can even be done- for the Tai-Kadai's reasons and mechanism of migration. One thing is certain, though: It's unlikely for it to converge. IE, the Thai and Myanma in Thailand and Burma should be present in somewhat different areas at the very least, perhaps with a remnant kingdom between the two groups. More likely, they would have done something different- if not a completely different direction, it's quite plausible for them to have wiped out the Viet before they were really Viet. It's also possible for the group to have died out rather then spreading, or to have stayed in place. They're a big wildcard, basically, but it's likely they would have done thing different even with a tiny POD in Tibet, much less the major things you have done.
Note also that alternate migrations mean alternate campsite locations, which mean altered weather, in turn butterflying the existence of every single noteworthy person in the Americas who was born more than 30 years after the POD. Basically? No Inca, no Aztec. Surviving Mississippians? Surviving Amazonians? Sure, that's plausible. Different Peruvian and Mexican empires? Sure, that's plausible, but it will be different uniting factions in all probability, with different borders.
