Probably not worth the risk doing that more than once though - the lowest I've seen a normal (first class) one-way IAD-LAX go for is $800ish, so if this $257 pricing is not intentional, they might put a stop to rampant hidden-city/throwaway ticketing when they notice a pattern.

I would note though, that one can comply with every explicit letter of VX's rules and do the following if you wanted a full day/night in LAX/SFO:
-book a morning IAD-LAX/SFO with a short layover before the LAX/SFO-DAL flight
-upon arrival at LAX/SFO, stop by the desk on the way out and pay $35 to change the LAX/SFO flight to the last flight of the day to DAL
-miss that flight, call and standby for the first flight to DAL the next morning (no further standby or same-day confirmed change is available beyond that)