I know the OP is talking about his milling vise, but I see no reason why my solution for my bench vise can not also apply there, at least in some situations.
Also, there is no real reason why a vise, milling or otherwise, can not be used without the jaws. But I would limit such use to occasional situations where it solves some particular problem as it did in this case. Tools are tools and are made to be used. In a pro shop, they also need to earn their cost. A hammer is made to hit things and it gets some wear with each nail or other thing that it hits. But that IS it's purpose and I have hammers that are at least 50 years old and still going strong.
I look at vise jaws as doing two things for you: first they are consumable and replaceable so they can extend the life of the vise. Perhaps your particular job did no damage to the jaws, but even with care, my vises often show signs of wear and tear after years of use and at least 90% of that wear and tear is on the removable, replaceable jaws. I have also seen much damage in photos of vises on the web and again, most of it is to the jaws. So, they do work in this manner.
The second thing that they do for you is to provide a way for using the proper type of jaws for each item or job. Thus, I have several sets of jaws for my bench vise: aluminum, soft, wood, and felt at the present time. On my bench vise, I can pick the ones that will be best for a given task or work piece. The problem with this is the time needed to remove and replace the OEM jaws that are mounted with screws. Two screws per jaw is not an excessive amount, but still it takes time and that is often a reason for NOT matching the jaws to the task.
I have purchased and made various jaws with magnetic mounting so that the jaw change is very fast, it takes only a few seconds. So, unlike screw mounted jaws, my magnetic jaws are actually used very frequently; I almost always choose the one that is best suited to the task at hand.
With a bit of care and planed machining, a set of magnetic jaws for a milling vise could be made to be at least +/- 0.001" across their faces. The only problem would be how to clamp them in place while making the truing cuts. But if only a step at the top is to be used, then that is not a problem.
I have only recently purchased a couple of proper milling vises for my mill and have not gotten around to making any accessories yet, except for a pair of excellent, contoured table covers to use with them to keep the chips out of the Tee slots. I will probably at least make a pair of aluminum jaws for one of them and it will probably be magnetic. I have table mounted stops, but want a vise mounted one also.
