I would transplant in winter, prior to bud break, while the plants are still dormant. Make sure to retain as large a root system as you can. Some grapevines have impressively large root systems. I would also place some 'Biovam' (sold in Spokane / they are easy to find on a web search) in the new planting hole.

Keep in mind that if the site to where you are replanting has had grapevines growing there in the past 3 years, your transplant will likely not be successful, as grapevines apparently secrete a substance that is damaging to new plantings.

If any body would like to buy wine grapevine cuttings (cheap way to establish your own vineyard), see www.hesedvineyards.webs.com Now taking orders for Spring 2009.

I hope this is helpful.

-Bret

Here is a link that might be useful: Hesed Vineyards (cuttings available)