Billy,
As with most things, the answer is, "It depends..." Generally speaking, you get a much longer travel when "coming down" on a part held in a vise. You really only have "side of the cutter" access at the ends of your vise -- a much shorter "throw" for your cut. A really good, rigid, heavy vise helps.
Once again generally speaking, you want to use the largest cutter your have when dressing a surface (within the limits of your mill). 1/2 inch = 12.7 mm -- so you won't really cover the width in one pass with your 12 mm endmill. Even if you could, the edges of the cut would be mostly tangential to your endmill which (generally) leaves a poorer finish for your cut and raises a bigger burr. You will almost certainly need to make two passes to face your piece well. If your mill is rigid enough. make the cut such that the cutter pushes the burr "into" the piece and your cuts overlap by (about) 20% of the cutter diameter. This will leave less clean-up and deburring filing for you to do.
You do not say if you are working with extruded bar or plate material. Extruded aluminum bar rarely has good, flat surfaces. Rolled plate generally has very good, flat surfaces. If you have extruded bar, you will probably want to "dress" all four surface. If the extrusion has been well stress relieved, this is not a problem. However, there is a lot of poorly extruded Chinese (and, to a lessor degree, Indian) aluminum extruded bar being sold. Many of these are not stress relieved (regardless of what the material ID says). When you cut one edge, the bar will "bow up" (towards the cut face) to relieve the stresses (it may take a couple of days to appear). This can be very disheartening. The solution is to place your pieces of bar in an oven, heat them up to something greater than 440°F (225°C), hold them at temperature for 20 minutes per inch (25 mm) of thickness plus 10 minutes, and let them cool in the oven to room temperature. This will relieve stresses without damaging the temper. It is a good idea to let such "home stress relieved" material sit for at least 24 hours before machining as it can take up to three days for the full temper to return after stress relieving.
Two weeks ago I got called into a company that makes a medical device. One of the items supporting this device is a pin-spanner to tighten a cover. The pin-spanner is 8 inches wide. Whomever they got to make this used extruded aluminum bar and "dressed" it (both sides) to thickness. The day the spanners were delivered, everything fit perfectly. Several days later, the bar bowed and the pins no longer fit in the holes. I see this all too often.
Additionally, once you have trued one surface, you want to "roll" it in your vise such that the trued surface is against your "fixed" jaw. Make your second cut and, assuming your vise is true, you will have a good 90° surface pair. Dress the remaining surfaces and you should end up with a nicely sized, truly square blank to base the rest of your work on.
