Do you mean your fare class is "N"? United doesn't use "E" code as a fare class. "N" is a basic economy fare.

In all honesty, United is most likely to IDB after all the pax on board have filled up the seats and the last few people haven't boarded. However, they will always solicit VDB passengers first. On a short route like SFO-LAX they will always find takers because there are literally a ton of flights between those two cities; not just on UA but also on DL, Virgin, etc. etc.

For that route I would think the chances of IDB are about 0.0001%. I wouldn't even have it cross your mind.

-RM