The title says it all: the shuttle bus seems to start at 6 am but I need transport between terminals at 5 am. Can you take a taxi, and if so how much?
It's less than a half a km. It's easier to just follow the signs and walk it. It's a small airport.
I doubt any taxi will take you as they wait a long time for customers and taxis are very cheap in Hanoi, unless of course you agree to say 100,000 VND or something - money talks. It's a little unfair to expect meter usage in my opinion regardless of the "law".