Rabbits have shown themselves to be extremely adaptable to human environments. If you have rabbits in your Heirloom / Open Pollinated Vegetable Garden, you have lots of rabbits. They mate several times a year and each litter can have six or seven little bunnies. You don't need a calculator to figure out the kind of tremendous possibilities that we have here. If you have a Heirloom / Open Pollinated Vegetable Garden it means a lot of rabbits in your garden.
Rabbits like to feed at twilight, but they don't always follow these rules. I've seen them feeding at dawn and in the middle of the day. They'll seek out a Heirloom / Open Pollinated Vegetable Garden in the spring when their normal food supplies aren't as available. Rabbit damage can be tremendous in the spring. Just a few bites into your Heirloom / Open Pollinated broccoli, lettuce, cabbage, or cauliflower transplants can set them back for weeks. If they nibble the center bud the plants wont ever produce.
Barriers: Chicken wire fencing can rabbit proof your Heirloom / Open Pollinated Vegetable Garden. Bury the bottom of the fence at least 6" below the surface, the top need to be at least 2 ft tall.
Repellants: In a small garden, sprinkle black pepper on your transplants in the evening. Rabbits sniff everything and after a sneezing fit they'll move on. If you can find their path to the Vegetable Garden, you can scatter some moth crystals along it.
Blood meal: Some folks say if you sprinkle blood meal at the edge of the garden and on some plants, rabbits will avoid your Heirloom / Open Pollinated Vegetable Garden. I guess if they smell blood they think someone means business.
Eliminate Daytime Cover: One way to reduce the rabbit population in the yard is to remove brush piles, and others form of daytime cover. Clear out overgrown walls, fences, and ditches. Rabbits like alot of cover and if it is not there it will discourage rabbits from hanging around. If you live next to a woods you are limited to what you can do to eliminate cover, fortunately, nearby big trees may help, since they will encourage predators such as owls and hawks.
Pets: A good dog or cat can be priceless when it comes to keeping rabbits away. It's pretty comical to see a cat carrying a rabbit as big as it is, across your front yard.
Elmer Fudd method: I hear rabbits taste pretty good!