It's an excellent question but my research hasn't turned up anything specific about lingering effects of sunflower allelopathy on subsequent plantings of vegetables. Whatever the lingering effects, if any, they would interact with different vegetables differently as the allelopathy biochemicals are plant specific.

The allelopathy biochemicals will degrade over time even if you do nothing. I suggest you start your project with a soil test and just concentrate on improving your soil from this starting point. You can obtain a test here: http://www.soiltestinglab.colostate.edu/ 

Anecdotally, I have a birdfeeder that I have filled with black oil sunflower seeds for close to three decades; the ground underneath it is littered in sunflower hulls inches deep and is virtually bare. I attribute it partly to compaction and the fact I don't water the area but also to the allelopathic properties of the sunflower hulls. In another part of my Denver garden where I grow natives, I have sunflowers begun when a bird dropped a seed. They pop up and I leave them because birds and pollinators love them. They haven't appeared to have had any deleterious effects on the surrounding woody and herbaceous plants.

I don't know what you can glean from that anecdote other than you might need to do some trial n' error. If you find you can't grow germinate seeds in your soil or the seedlings fail to thrive, you could raise your beds and import some soil.