VS will apply the miles and $ as on the "open" ticket and you will need to cough up the difference.

VS does have a "no fare difference" policy for small fare differences, but that is only in place for revenue tickets, and only for small, not material, differences (as is the case here with DL awards having went up by as much as 3 fold precisely here). For award tickets change, even if, say,10 cents difference in the fees and the mileage is the same, VS will still charge you that before reissuing the ticket.

IMO better to simply refund and redeposit your tickets with canceled flights. Less messy later. Plus, you may get to find out how much did VS actually pay to DL for your award seats (which is always fun to know, at least I think so), as the VS refund confirmations will occasionally show the value, in several hundreds of dollars normally, that it has paid to DL for the ticket...but no, you don't get that refunded back, you only get the fees/taxes you actually paid, DL presumably gets the rest.