In a 3-quart rectangular baking dish combine vinegar, oil, herbes de Provence, salt, and pepper. Add pork tenderloins to baking dish, turning to coat. Cover and chill for 30 minutes or up to 2 hours, turning meat occasionally.
For a charcoal grill, arrange hot coals around a drip pan. Test for medium-hot heat above the pan. Place tenderloins on the grill rack over drip pan. Cover and grill for 30 to 35 minutes or until an instant-read thermometer inserted in centers of tenderloins registers 155 degrees F, brushing with half of the Red Wine BBQ Sauce during the last 15 minutes of grilling. (For a gas grill, preheat grill. Reduce heat to medium-high. Adjust for indirect cooking. Grill as above.)
Transfer pork tenderloins to a serving plate. Cover with foil; let stand for 15 minutes.
Snip grapes into small clusters, keeping grapes on the stems. Grill grapes directly over coals for 3 to 4 minutes or until grill marks are visible and grapes are plump, turning once halfway through grilling. (For a gas grill, place grapes on grill rack over heat. Cover and grill as above.) Remove from heat; set aside.
To serve, bias-slice pork into 1/2-inch-thick medallions. Serve with the remaining Red Wine BBQ Sauce and grapes.
Red Wine BBQ Sauce
In a medium saucepan bring wine to boiling over high heat. Reduce heat; simmer, uncovered, about 10 minutes or until reduced by half. Add broth, barbecue sauce, and mustard; simmer about 15 minutes or until reduced to 1 cup. Reduce heat to low and slowly whisk in butter, 1 tablespoon at a time, until combined. Season with salt and pepper. Makes 1 cup.