Virginia pays UIM without credit for the other driver's coverage unless you elected otherwise, sets an order of payment among policies, and bars recovery for any negligence
Virginia's rules are in Va. Code 38.2-2206. The deadline is in 8.01-243 and the minority tolling rule in 8.01-229.
Read how stacked and single-limit coverage differ in Virginia, check what section 38.2-2206 says, or see how Virginia UM and UIM work.