Vacation and Employee Classification

What is the name of your state? MI

Our company has a standard vacation policy where employees earn vacation and sick time after so many of years of service (i.e one week after one year, two weeks after three years, etc.). We have sales people in the office that are out often. Since is it nearly impossible to determine if they are out of the office on company business or taking time off for non-business reason, we would like to make the exempt from this policy (allowing them to take off when they want as long as they produce). Is this legal?

Sure, although I'm not sure it's a good idea to have a limitless vacation policy for your sales employees. But if you want to do so for them, and just for them, you are completely free to do so.