Once you’re in HCMC, it’s a $6 or $7 taxi ride to the airport (take Mai Linh taxis if you don’t want to experience the joys of rigged Vietnamese taxi meters). There are a dozen daily flights that fly the HCMC-Hanoi route, priced as low as $40.Vietnam Airlines tickets is the most reliable but most expensive and they charge extra for one way fares. A better option is Jetstar Vietnam. They’re cheap and while Vietnam expats like to gripe about them, I’ve had pretty good experiences with them.
There’s a newcomer on the scene, Vietjet Air. I’d strongly recommend against using them. Although they have 6 flights a day on this route, they usually combine them to fill planes, and only run a few per day. Both of my Vietjet Air flights were cancelled, and I was moved onto one of the later flights. On the way there, it was only an hour delay. But on the way back it was a 5 hour delay. Definitely not worth the $5 savings. With any of these airlines, though, expect some delays and don’t schedule tight transfers.