The sales are acceptable in the sense that Sony has no choice but to accept them.
I think that when the Vita starts to fulfill the promises it made before launch, it will be okay. When we get
-Vita vs. PS3 play in games that matter
-Good PS3 games that let you carry on your saves in their Vita version
-PS3 titles that give you a free copy of the Vita version
-PS1 compatibility
-Wii U styled gameplay in select PS3 titles
Then, people will actually pony up the money for the device. Right now, as much as I love my Vita, it's really just a PSP with better graphics. It has so many other features and abilities that just aren't being used yet (if ever). Great hardware that's just going to waste, in my opinion.