Delta should interline your bag to VS. If you are on the same ticket, it will happen automatically. If you are on separate tickets, you will have to show them your VS reservation at check-in.

An 8 hour layover is plenty of time for your bag to connect (at Heathrow it will actually go to a special storage facility for long-layover bags). If you do something like this in the future, note that on a 12+ hour layover, Delta requires you to pick up your bag at the baggage claim and re-check it. There's a thread about short-checked bags on DL around here that details that.