I assume your referring to the Vise-Grip 4LW model with the notch in the fixed jaw. It is a great brake line tool.

The trick to loosening brake hoses is this: You want to hold the flare fitting on the hard line immobile and apply all the turning torque to the hose fitting. Keep in mind that the tightened hose fitting clamps the flare in the line to the hard-line fitting. That makes the hard line and the flare fitting act like it's one piece. Apply turning torque to the flare fitting and you run the risk of twisting the hard line.

I often cut the hose off so I can fit a deep 6-point socket on the hose fitting. Then I use a 3/8-inch drive breaker bar. You can really crank on the hose fitting that way.