Need to keep your parallel bars in place when using your milling vise?  If the answer is yes, then you need these parallel bar holders. You'll need to print four of them, two left and two right. 

Magnets are required to make them work. The size is 3/8" x 1/8" magnets. They can be found here https://a.co/d/2S0uiJ7. I recommend Super Strong magnets, at least grade N42 to keep them in place. Use your vise to press fit them into the printed part.