Speaking from experience, if you wait until the soil is warm enough, then plant the seed, you will have veggies almost as soon as you would by buying the plants. Seeds planted shallow will sprout and come up in about three days. They will start to grow immediately. Transplanted plants have to adjust. My oldest son was buying plants. I told him what I just said but he wouldn't listen. Last year, he had to replant and the seeds sprouted and came up in three days. So now he says he will plant the seeds. Mother knows best.
Well, that maybe true for a Dixie Chick, but for us up yonder, by the time seeds in the ground sprout, a planted former potted plant, is starting to bear fruit.
One year I used trenches to make my compost. I threw a bunch of rotten tomatoes in it. I decided to not use that system anymore and left the long hilled row until I felt like tilling it and planting grass over it. About the middle of June when my other transplanted tomatoes were turning color and a couple weeks off harvest, numerous tomato plants sprouted on that compost.
Out of morbid curiosity, I decided to leave them. They were all from hybrid varieties so I expected a real freak show, if they bore fruit at all before frost. We had a long summer that year and they made a few reds, but were loaded with greens.
I had no use for green tomatoes, fried or no, but one old time ham radio guy was coming from the coast to buy some of my gear I was selling off. I told him about the green tomatoes and he said his, Southern Belle, neighbor lady wanted them. I told him to bring 20 of his 6 gal plastic buckets he got from a Mongolian restaurant, the same restaurant where we went to lunch years earlier. I mean strange funny dressed men dancing around a hot cone, flipping food, without regard for separation. Food that was laid out on a table like a swap meet, so you could pick what you wanted. I wasn't impressed that my careful selections, avoiding foods that made me gag, like octopus, I can't spell the fancy name for it this early in the AM, were ignored with such reckless abandon.
We filled those buckets and that was only half of what there was. He denied any hanky panky with the Southern Belle but he wasn't fooling me. He was also peeved that one tuning unit he wanted, I had sold to Joe Walsh that was also an old time ham radio buff. I pointed out that not only had Joe spoke for it months earlier during one of our roundtable discussions, but also had he ever stood on a corner in Winslow, Arizona and seen a girl my lord, in a flat bed Ford? No, he had not, so there!
Anyhoo, the Southern Belle green fried tomato lover, was very appreciative of the haul he brought her. The moral of this story is, the self seeded plants were two months behind the ones I set out from my greenhouse.