The most important thing about any console is that it can play a game released up to 6 years after you bought the device as well as games you bought any of the years before. So let's take the analogy for iPhones. If you had the money to buy a new iPhone every year, and developers could make a game every year that takes full advantage of that new iPhone, then yes, the iPhone would have better games every time.
But the reality is that not everyone can buy a new iPhone every year, and not every developer can therefore afford to make a game specifically for that new hardware every year. So you will get compromises where games are developed to work with older generations as well as new, and people with older generations of iPhone will get a lesser experience, while people with newer generations of iPhones will get a sub-optimal experience.
This is all not a big problem necessarily. But it is simply a lot like PC vs consoles - both have their advantages.
The big advantage of the console setup, and the Vita in this case, is obviously that you can buy a Vita now for 199 euro (and potentially less as time goes on, or with more/cheaper memory cards etc.), and every game released in it over at least the next five years will run great, and as the device becomes better known by developers, you'll even see some graphical improvements that you didn't have to upgrade your hadware for.
Yes, at the end if the cycle it will start looking (significantly at times) less good than iPhone games etc. but even then you'll have great value for money and, let's not forget, analog sticks