I don't exactly have a portable vise that will keep the rifle perfectly level, but I use a bubble level on the rail of my precision rifles. Here is my process:
I place a Harris swivel bipod on the rifle so that wherever I place the rifle, I can use the swivel to ensure the rifle is level side to side. Then I place a rear bag under the butt of the rifle until it is mostly level front to back. This isn't totally necessary because we are interested in level side to side, but it gives me peace of mind.
Then I place my bubble level on the rail of the rifle and double check it with a torpedo level a carpenter might use, the little small kind.
One I'm happy with the rail mounted bubble level, I take the torpedo level off and mount the scope rings with the scope in them loosely to the rail. I place the torpedo level on the elevation turret and ensure that the scope is level and the rail mounted bubble level is still level also.
I then tighten the ring cap screws in an alternating manner while ensuring tha the scope remains level.
So now the scope is mounted to the rifle and the rail mounted level is showing level as well. Now I hang a plumb line out at whatever distance I can see it well and check that when the rail level is showing level, that my crosshairs are plumb with the line. I'm usually so close to perfectly level that it isn't worth trying to change anything. Hope that helps you out.