I strongly advise against putting down a cloth weed barrier. Your plants will do much better if they have access the the soil below. With a barrier, you're just making a giant container. I've been spending the last 5 years covering about 5,000 sq ft of weedy lawn with garden beds. I cut the grass short with a lawn mower, lay down cardboard on top of it, cover with 3-4 inches of soil and mulch. I get very few weeds popping through. Definitely with 18" of soil over the top of your lawn, the grass won't survive. Line the bottom with cardboard or paper grocery bags if you'd like, but be sure it's something that will decompose. Dump your dirt on top. Many vegetable plants, such as tomatoes, have deep roots, and that decomposed grass will give you a good bounty later in the summer.

If the can was rinsed, it may not be a set back. If you'd realized the mistake immediately, watering with a hose, soft nozzle spray (which I would have been doing with a newly seeded lawn anyway) would have been all you could have done to try to remedy. Now, it's wait and see if you'll be reseeding. But I have to ask, why did you have Roundup in a watering can (and a leaking one at that), it's applied as a spray to foliage, not watered into the ground or poured on - should only be in spraying type container. Please don't rely on memory to mark your products or applicators. Get a can of spray paint, red for anything used for herbicides or blue for insecticides if you have those and in the future keep them separate. If you are using and mixing concentrates, it doesn't hurt to mark the product container too, some of these companies use same color/size bottles for everything and you can mix something other than you intended. Put a blotch of bright paint on any applicator or product that would be harmful - to you or your plants.

Contrary to common wisdom, putting gravel under the soil will make drainage worse. So save all that labor for a different project. Why do you think it stays so wet? What kind of soil or soil mix is in the planter? How deep is it?

"I assume there would be health concerns with the Weed & Feed chemicals getting into a vegetable garden. What should I do to address these concerns?" I would guess that by the time the cardboard disintegrates enough to let plant roots through into the existing soil the weed and feed would be a non issue. If you are really worried, send an email to Scott's and see what they say. Rodney