What is the best way to cut metal siding for soffit vents accurately?
If you're trying to cut in place, I'd personally go with drilling an initial hole, and then cutting the shape of the vent with a "sheet metal nibbler".



The nibbler will let you cut from one side, as there's a small bit that you insert into the hole, and sheers off a small roll of material (somewhere near 1/8", depending on the exact pair).  You can also get powered ones, but they're fairly pricy, and the weight might be annoying for overhead work.

You might want to check with local rental shops, to see what they have.  (the increased weight might be worthwhile as you won't have to hold your arms up as long)