I heard that some question whether vietjet really has financing in place for this order, but I'd assume airbus also does some due diligence before making big announcements. The plans have existed for a while, they've had enough time to get things in place, so I think its quite likely that this is backed by more than just hot air.
As for the new airport in SGN, there is some bickering as always and everywhere, some suggested expanding TSN to the area of the golf course to the north and claim it would be cheaper than the 8 billion usd new project, but as far as I know its going ahead, partially backed by "official development aid" money, and plenty of "consultants" (international tax dollars at work...)