It depends on the vegetables the flies are on. I thought I was fine with the flies on my spinach only to find they were spinach leaf miners. I'm still fighting them.

It depends is the answer I can give. Of course I'm no expert. Good luck and watch them closely for eggs under the leaves.

Identify the flies and you'll have your answer.

Otherwise it is all guesswork and your guess and my guess and sinfonian's guess is as good as anyone's.

Dan

I've been having a similar issue with flies. I bought a few strips of whitefly paper, cut them into small strips & put them out last night using bent up paperclips I stuck in the soil. They are packed with flies today... Being able to see them close up w/o them flying away, I can see they're aphids (I originally didn't think they were aphids). I did this because I was having a hard time getting a good pic...

Maybe you can do this to a) reduce their numbers, and b)prevent them from flying away so you can identify or take a pic and post on the pest/disease section.

Here is a link that might be useful: Gardening With Care