Yes I have harvested many acres of hairy vetch here in my area, which is of course western oregon.

 If you have summer rains or moisture in the soil, it will continue to bloom till the moisture runs out or a freeze, whichever comes first.

 Here we try to lazily throw the blooms into 3 "sets". Usually the second set is the one we choose to harvest, but you'll have to choose for yourself after scouting the field.

 Of course there will be different ways to harvest vetch in other areas or on other farms. However our prefered method, which most locals have used for many decades, is to swathe the vetch for drydown. If we have even a slight chance of cloudy, cooler weather coming up, we swathe just ahead of it, let it dry down during the cool spell, and combine it as the heat wave hits.

 Again, I'm sure someone will have another method for decision making on when to swathe, but I bust open the pods of the set I choose, and look at the seed inside. If there are any black specs starting to appear on the majority of the seeds, I swathe. It seems that once the black specking appears, the seed is mature enough to make a viable seed if cut off from the nutrient supply. If no specking has appeared, the seed may dent and shrivel. Again, it seems to be an arbitrary decision.

 Once cut and some dry down has begun, I will check on the rows continuously. Green but dry vines and pods will fly through the machine. Some of the seed will still have some green color to it, but if it is storable moisture, or you have access to a cleaner that can scalp it quickly, you might be able to combine a day sooner as the chaff/mog is what usually causes storage issues while waiting to be cleaned. If the weather turns hot and windy on the windrows, you may be s.o.l. I have asked about the "pod seal" sprays locally, but recieved unfavorable replies. Maybe your area has better luck with it. ?

 The combine usually needs a slow cylinder/rotor, but reasonable tight concave clearence. The seed is fragile, but the pods can wrap the seed as they are a twisted like pod half once opened. Sort of a compromise between the fragile nature of soybeans and the roll up ability of a corn husk. The matted, greener vines and any weed matter can carry the seed out the back as well.