This might help you find more information. The toxicity you're referring to when one plant secretes a substance that is toxic to other plants is called "allelopathy".
I did a quick search for allelopthic forsythia and it turns out it is. This page lists some info and some of the plants that won't work well with forsythia. http://gardening.about.com/od/gardenproblems/a/What-Is-Allelopathy.htm
It's not a complete list so you might want to look for specific plants you want to add.
Update: Out of curiosity I did some specific searches trying to find any info regarding how forsythia might impact vegetables and I can't find much. It makes sense. Landscape plants are usually not planted with vegetable crops so it would be unlikely for a researcher to have the incentive to look into it.
You may want to look into the families of plants that forsythia can deter and compare it to the family of vegetables you plan to plant.
For example goldenrod is affected by forsythia. It is in the aseraceae family. So is lettuce, sunflower, artichokes and marigold (not edible but good companion plant).
Black cherry is in the rosaceae family as are strawberries, apples, plums and peaches.
Now I'm curious. Hope you can post an update on how things went.