Clematis vitalba is considered an non-native invasive in many states. You may want to check to see if it is listed as such in your state before planting it. Why not plant a clematis terniflora or sweet autumn clematis if you want a fragrant white flowering clematis that just might be in flower in August in your zone. Like Marie said, I wouldn't expect too much the first year any clematis is in the ground as it is establishing its root system and flowering more than likely won't be its first priority.

The chalk hill web site has a list of clematis that will grow in partial shade. I have a hagley hybrid that is partly shaded and it does very well. I picked up a carnaby a couple of years ago and read that it fades badly in the sun so I have it in a spot where it is completely shaded during the day and gets about an hour of the last sunlight of the day. Here's a picture. You can see the trunk of the tree in my neighbors yard that shades it all day.

I would be more concerned about the roots of the oak trees. Are they LIVE oak or Laurel or ???? Oaks roots do not like to be disturbed or covered with extraa soil and they oft times suck all the nutrients from surrounding plants but,NOT all the time. I have sweet potato vine Marguerit doing ok and some elephant ears. I toss cosmo seeds around and they aome up anywhere the weed whacker doesnt find them,Whatever you choose plant small ones or seeds so you do not have to dig big holes. Its easier that way. I agreewith corar that even my veggie patch gets some shade or the stuff would all fry. Hang orchids and pretties from those shady spots also they are a cinch.

Most fruits want full sun to set and ripen the fruits or berries. You could try something like rhubarb or some type of more woodland species like huckleberries (Vaccinium ovatum or parvifolium) but you won't have much luck with things like blueberries, raspberries, strawberries or grapes - they need at least 6 hours, would prefer more.