Make a step key to fit the slot. Also make step keys for the horizontal orientation. Make them accurate and tight fitting and you don't have to mess around setting it up each time.
Edit: David beat me to it. Need to learn to think and type faster... 8^)
Second edit: Use the same method for mounting the vise. Saves lots of fooling around setting it up with jaws parallel to the table.
Third edit: Welcome to the forum!
Yeah I think you are right. I think I also doubt my ability to get them right, so that everything is right on when they are mounted. Will give it a shot of course.
And thanks.