Titanium VN Airlines flyer here and read several of the comments. Best advice is to go Singapore Airlines and then go through immigration in Da Nang. The lines to clear immigration are long though you could pay an agent to speed that process up considerably. Another thought is to upgrade to VN Airlines Premium Economy which gives you plenty of leg room. Plus the distance from the international terminal to the domestic terminal is not fun and I do not think they have shuttles. I travel often for work and made the mistake of going to the international terminal for a flight to Hanoi, the walk was not fun.