Why is the Vita not selling well?
When it comes to handheld consoles it is all about handheld games...
If it was all the other features then people say "why would I buy a handheld rather than a tablet or smart phone"... Sure handhelds are generally cheaper, but tablets and smart phones have the plethora of app support.
So, when it comes to games... Nintendo is associated with handheld franchises such as Pokémon, Mario, etc. While Sony's name is associated with console franchises such as God of War, Unchartered, Killzone, etc... The Nintendo games are well suited to handheld devices while the Sony franchises are strong with full fledged controllers. (Although I do really love the God of War games on Sony Handhelds)...
I think the Vita devices are better than the DS devices. But, there are not many very good handheld franchises in the Sony world. While Nintendo's name is almost synonymous with handheld franchises.