I think there is a few problems here. First, if you (or your mom) didn't want the VDB compensation offered, you shouldn't have relinquished your seat--trying to change the terms of the deal afterwards seems a little dishonest to me.

Second, even if UA had a policy to allow exchange of these vouchers, finding someone who actually knows and would instruct you on how to do so might be difficult to find. If it isn't policy, your chances that someone will go above and beyond are very low.

Personally, I just refuse RT vouchers for VDB compensation--they are of no use to me (except as CC fodder, I guess). If the GA is desperate, they'll give in and offer vouchers.