Apply insectidal soap to control adults to the undersurfaces of the newer leaves. If the problem is severe, spray plants every three days. Another suggestions is to spray summer oil on woody ornamentals (horticultural oils for growing season use), that can tolerate summer oils (check label)
this was a response from UBC botanicals to someone who had asked the same question. I have also heard of a half water half milk solution, but haven't had the guts to try that yet. what about citronella? just a thought. we have grapevines too and all I ever watch for is skeletonizers.

You can trap the adult whiteflies with yellow sticky traps. For whatever reason, they are attracted to the color yellow and get stuck breaking the cycle. Purchase ready-to-use traps or make your own with plastic yellow plates smeared with something like Tanglefoot. Vaseline will work but melts in warm weather. Staple or nail the plate on a wooden stick and place near your grapes. Voila!!!

Here is a link that might be useful: Whitefly info and control from UC Davis