I believe that DL, as the operating carrier depositing you at LAX, would be the carrier legally responsible for getting you to your destination. However, VA staff at LAX should have helped you as it was their ticket stock and would be much easier for them to touch/change your ticket to get you to DEN.

VA places their code on Frontier flights? Interesting. Did you go to the Frontier gate, explain the situation, and ask if you could standby for early flights? They're usually pretty helpful in these cases.