I can't guarantee this will work but the following solution might be plausible. The reason why they won't put you on the flight two days earlier is probably because anything more than 24 hours is a stopover and you are only usually allowed one stopover on a return redemption. You are already taking that between your SFO and Vegas flights. What routing are you taking back from Vegas? If you are flying direct Vegas to Chicago then you could get two separate tickets to and from SFO as one way redemptions. SFO-LAS_ORD-ZHR would in all liklihood be a valid route for the return leg and on one way redemptions you are allowed a stopover in each directon. You can then have a free stopover in ORD on the outward and Vegas on the return. If they reissue the ticket though there is a risk someone else takes the seats between then cancelling the return and reissuing them. PTS should be Plan A as already stated. This is only a back up plan!