Only the orange counts, not the camo'd parts. If it's simple, dark blotches, it's a lot easier than some of the other patterns, but still a bit of a pain. What I would suggest is to get a sheet of graph paper, and trace over the camo pattern. Then you can count and divide the number of squares in the orange by the total number of squares on the sheet. That will give you a percentage of orange. Then measure the total area of the vest, and multiply by the percentage to figure out just how many inches of actual orange are on it.
Personally, I agree with the other poster that it would have to be a very small vest not to be legal even with camo. This would likely give you a headache and waste a lot of time. But that's how you'd do it.