Hi- our viticulture specialist shared this information with us:

• The classic cultivar used in the US is "Sultana" ("Thompson Seedless") but regretfully it is not typically available in the eastern US.
• Look for cultivars that have leaves that are more flexible - that means vinifera, then hybrids, and the least desirable would be American cvs ('Concord', etc.), as they are thicker and have more pubescence on the underside, making them tougher. If you have a choice, in general, the Italian cvs ('Sangiovese') tend to have larger leaves than the French ('Cabernets').
• Pick leaves at least the size of the palm of your hand, preferably the last fully expanded leaf toward the shoot tip (just below the new growth), with light green color. They will be more tender than older leaves toward the bottom of the shoot closer to the cluster.
• The best time is late spring (May or June) during the most rapid growth stage of the vine.

Vitis vinifera (European wine grapes) are more disease-prone and less hardy than American table grapes. You may want to also contact grape and small fruit nurseries for their recommendations.

Jon