You will probably be alright with the grape, it's soft though so it will rot fairly quick as opposed to hardwoods. Are there not any rivers, forest in the area that you could find driftwood? I've used all kinds and never had a problem, grape, walnut, cedar, hedge apple (osage orange), I've heard people say to avoid these but I've got all of them in different tanks with no problem. The important thing is if it has been aged outside for a long time.
As for the tannins, Purgin (sp?) is a great product. I've got a big piece of walnut in a 75 gallon that's too big to boil, charcoal would only slightly touch the tannins it produced, we are talking Rio Negro blackwater here. Put a pack of Purgin in the filter on Friday, by Sunday it was crystal clear, plus you can recharge the Purgin and reuse it.
I don't use green wood though, everything I use has been dead for years usually. If you want to use the grape wood, you could try it, remove the bark, dry it, then soak it. I doubt it would hurt anything. One thing with newer wood, you'll probably get mold blooms, overnight it will start growing hair, it won't hurt your fish and you can either take it out and scrub it or put a bristlenose or two in the tank and they will eat it.
Drying in the oven: You can probably do it this way but you might want to use a lower temp. At 250 there will be some checking (cracks) that develop in the wood from being dried quickly. OR you can seal the ends with Elmers glue so the moisture leaves from the sides, not the ends (where checks start). Oh, and leave the door cracked to let the moisture escape. Good luck!