I'm not sure if you've seen an old thread of mine somewhere in the handgun area (my first 1911 build), but I have done a number of dremel builds since off 80% frames. Now I have tried the setup you're going to do - and unless you or someone else has posted 100% success with that method I found it to be very unstable. For one thing, unless you use a mill, the cutter should be much thinner so you can make more than one run. That allows you to correct any small errors you may make. If you get a cutter that's the exact size or close, it leaves very little room for corrections.
Along those lines I found when trying the slide vise/press method the vise/press never held the frame true for the entire cut to be made. That's why when going the "poor man's route" I've always opted to using the dremel with cutoff discs. Sure, it takes longer and you go through tons of discs but you can have very good control over the depth and overall outcome of your cuts.