If you put preemergent down in the spring it has worn off by now. The St. Louis area has been pounded by some good storms this summer if I recall, so that doesn't help the longevity of the weed preventor.

Weed seeds can last in the soil for years. The preemergents and weed preventors don't necessarily kill the seeds. They halt the seeds when they begin to germinate. Spray the incoming weeds before they seed. Stop the crab grass before it seeds. Vines can be coming in from any where and may be beyond your control if it's spreading from a neighbor that doesn't do much with their yard. If you can stop the weeds before they seed, over time you will knock them out. You just have to stay on top of it.

If you are looking for specific chemicals to stop weeds with search the forum for the weed and you'll find it pretty fast.

Thanks James. The spray I got said to do no more than 2 treatments a year which is why I was confused.

Do you suggest spot spraying as often as weeds appear?

Depends on the label. If it says don't spray more than twice, then it's probably to protect the grass. None of the broad leaf sprays I use give that warning. My crabgrass spray might, but I've never had to spray more than once. Just make sure the concentration in your sprayer is correct or else you can kill the desired grass. I made that mistake once hoping to speed up my first lawn project a few years ago.

Get on a good pre-emergent program, it's the foundation of a weed free lawn:

http://lawniac.com/conquering-crabgrass-and-annual-weeds

Whatever breaks trough, hit with herbicide:

http://lawniac.com/broadleaf-weeds