I installed ATM Fee Saver app, it seems fairly accurate. It says Agribank charges VND 22k, if it's true, you may want to change a small amount of cash at the airport instead. I noted the spread was 2.4% for CAD, 0.6% for USD. They were probably the worst rates in all of Hanoi (though hotels may be worse), but I'd argue they're one of the best among airports worldwide.

If you happen to be staying in Old Quarter near famous gold shops, such as vang bac Kim Linh (upstairs), or Gian Son Gemstone, you may get better rates than no ATM fee cards: CAD was around 0.2%, USD was -0.1% (meaning they often gave 0.1% more VND than the chart on xe.com showed).