Oh please do not spray roundup where you want to grow veggies. Preen helps prevent seed germination, but doesn't kill already established roots. Try taking a small jar of roundup and sticking some of the live roots in it. The grass will absorb it and die, but the rest of the area is not affected. I know this is a little time consuming, but worth it.
I can really sympathize with you, bindweed is the bane of my garden, I have it everywhere and I hate it. The neighbors probably wonder why I am out there sticking it's roots in jars and my son just laughs and calls me the crazy lady, but it works.