I'm skeptical this would work. With misconnects or no shows, the overbooks may already decrease. What may weigh in your favor, however, is the fact that the 10pm FRA and LHR flights are not operating today. In other words, if the airline wants to get folks off of your flight and onto another without delaying them a day, they just may have to put folks on an earlier flight. BTW, a $300 voucher for a VDB on an international flight is kind of a low ball offer.