The vetch will not release its N until it decays... but since it produces most of its own, neither will it steal much N from the corn. Given how rampant it is, the same may not be true of other nutrients... and it is very water-hungry. Of greater consideration may be the fact that vetch can easily become an obnoxious weed. I would never let it seed in a plot used for vegetables. Sunnfarm's idea of late-planting has merit, since the vetch could be plowed under before it seeds, and would restore some of the N depleted by the corn. I guess my question is... why vetch??? Why not something which not only improves the soil, but is edible? Pole beans were planted with corn by the Native Americans, as part of the "three sisters" method. Granted, that was using tall, sturdy field corn, not the shorter sweet corn grown by most gardeners. Tall pole beans would over-power most sweet corn. However, weaker climbers such as "half runners" & some of the "cornfield" bean varieties should do well. I have grown several semi-pole beans that might do well for that purpose. Cowpeas (a.k.a. Southern peas, or field peas) have also been successfully grown with corn by several GW members. Again, you would want to use some of the less rampant varieties.

To temper the invasive statement, that quality depends greatly on conditions, climate, soil etc. If you let it drop pods, then it's reasonable to expect proliferation. It also spreads by stolon. You could try buckwheat; though not a legume thus not N fixing, will grow on almost any soil. If you let it drop seed it will be back, but it's very easy to control and provides lots of green material.

The problem with winter cover crops around here is winter herds of deer. We used to plant winter wheat and rye, vetch etc. But the deer didn't go away when the cc was plowed over, but came back to dine on lettuce, spinach and newly emerged green bean plants. We have a fence but only 4 feet high. After years of no problem with deer, I just last week lost a whole row of lettuce and another bed of radicchio. Next year our garden will get a 7 foot high fence and we may try it again.

A few days ago I pulled all the hairy vetch from a 4' by 12' bed where I'll soon be planting tomatoes. I had planted the vetch in the Fall of 2008, as a cover crop. All I know is that the kale and zucchini and mesclun and whatever else I had in that bed last year all grew really well after planting that hairy vetch. I never replanted any, but this year the vetch came back much much bigger than it had when I pulled it last year. So, it seems to be helping, and it seems to be invasive, but maybe not such a bad kind of invasive. Last year during the growing season I had to reach down and pull out a few little vetch vines, once in a while. No big deal. This year I'm going to use Ruth Stout's method of mulching with 8" of rotting hay, so even if the vetch tries to come back stronger it won't make it through the thickness.