The problem of Vietnam allying with China early is a manifestation of what we call 'Finlandizing'. Which is where the smaller state draws closer towards a power near it, that is so much more immensely powerful than it. The fact is, in the game, Vietnam seems to either ally with China, or be conquered by it. I have given Vietnam the rich Mekong province as a level 1 colony, and started them with 6 colonists. Which they usually use to improve Mekong, and this normally results in atleast the capitol province surviving the game. Actually to be honest I give most states 6 colonists to start the game, to represent the many settlements that were started by a variety of states and that failed during this time period.