I do vacuum work on alum all the time. Some stuff you wouldn't think would work. Flat stuff is great. I've even come up with a way of milling an outline INSIDE the vacuum area of the fixture without losing vacuum. (most of the time)
Here's a fixture for some antenna housings I did on vacuum with NO clamps or other work-holding except locating pins. Note there are 2 setups here, the bottom one you can't see as well, but it's also done with vacuum and no other work-holding.
Sky's the limit. I have a 60 gallon dual motor lag/lead system that runs vacuum lines to all 5 machines, including 4 axis tombstones as in the setup shown below.