Feeding the nation isn't hard. We could feed this nation five times over with the amount of food we produce. The problem is, we'd probably shrivel up and die, because wheat, corn and soy aren't enough to keep a population healthy. We could actually produce more nutrition and calories with a fraction of the land just by switching from corn to oats. But then you'd be producing less, and hence selling less. That's not as profitable!
And vegetables are luxuries in the same sense that "nutrition" is a luxury. Just because humanity has historically had a shit diet (since the adoption of agriculture) doesn't mean it's a great idea to stop eating vegetables, or produce less of them. We get plenty of calories already - too many in fact. And most of the ones we do get are more harmful than good.
It would be better to get people to eat more of those low-calorie vegetables as it would help stem the American obesity crisis. And as we've seen in this report, vegetables already contain fewer nutrients than they did before - a reason to consume more of them, not less!
A single bell pepper today costs something between $1 and $2 where I am. Imagine the government subsidizes bell peppers so that they cost $.50. As a single bell pepper has less than 50 calories, you couldn't base your diet on it, you would still need some more calorie-rich food to actually give you required energy. Since vegetables (except root vegetables) are not calorie-dense, you'd need to eat seeds (e.g. grains) or oils anyway.
That's why, if your goal is to enable the nation to feed itself, you should rather subsidize the staples, not luxury consumption.