I recently grabbed some flights BOS-LAS in early Aug - T fare out and X back... and next day Z opened up on the outbound for a great price so I rebooked into first for $266 more on a 6hr flight.

So... the fares are definitely there even at 3mo out, but guess it depends what days and where you are traveling from. In fact, the fares were a lot higher and just dropped so it could be that you're a bit outside of DL opening up lower buckets if the flights aren't busy. Aug isn't exactly peak/busy season for Vegas.