I agree, it will sell under 20m units.
At this pace, it will only beat the Dreamcast because it will be alive longer. It is most likely overtracked here and is not near the 3m mark yet. Unless sometime soon there will be some amazing system sellers, sales won't pick up and the Vita will have a very mediocre holiday season. Because there is still nothing substantial announced, any system seller is pure luck. Moreover, the more time it takes until something else is announced the less likely it is going to happen. I have a feeling this time will be here soon, the point of no return so to say. Especially when the supposed system seller that actually were there, didn't do much.
I'm no doomsayer, mind you, purely based on observation I just wouldn't bet any money on the thing.
Maybe it'll change after christmas, but I just don't see it.
Essay Pro