Hairy vetch, like all legumes and bean family plants absorbs nitrogen from the atmosphere. Nitrogen is crucial to healthy plant growth, it is the first number of the NPK ratio found on all fertilizers.
Plant hairy vetch in autumn or late summer at least 30 days before the first average frost date. Although most varieties are hardy and will survive at temperatures well below zero , it needs temperatures of at least 60° F to germinate. It also needs a little time to become established.
It can be started indoors ahead of time, but alas Legumes do not survive transplant very well so direct seeding is advisable. Broadcast the seed over the soil as per seed packet instructions, generally about 1/2 pound per 200 square feet, and cover with a half inch of soil and a tinkle of organic mulch. Be sure to water it well, don't wash the soil or seed away just water it well. Once established it well grow throughout the winter in moderate zones, you shant be harvesting at the winter holidays but the plants will continue to grow even though they may appear dormant at times subterranean growth continues.
After emerging in the fall, the seedlings grow vigorously up to the first frost and then appear to be dormant- at times even die back a tad. They are however using stored root carbohydrate reserves to survive over the winter.
Mow or plow it under come early spring. Try not to allow too many to go to seed if it is your intent to grow other crops there, as it has a tendency to become invasive like a weed.
The plants should reach about 2 - 3 feet in height, although some varieties attain a 6 feet of height/length or more.
If you get a late start that's still okay. It wont help as a winter cover crop in this scenario but is still workable. So long as the seedlings have not germinated they will go dormant come winter. In the Spring as the ground thaws, they germinate - at a slightly lower rate, and emerge.
If the seed has germinated before the first frost, it is likely it shant survive extended periods of cold weather as it did not have ample time to become established. Timing obviously is critical.
Hairy vetch can be used in many companion planting and rotation schemes and it benefits are multiple.