First off, I should note that Sony can't just get the third parties. Forget moneyhats. If Square Enix does not feel their their studios' time is well spent working on an original Vita RPG, they won't bother.
However, the Vita appeals to similar demographics as the PSP, and people who were interested in the PSP are probably those whose interest is easiest to catch. A replacement IP isn't going to be as big as the original, but it doesn't need to be. If it makes the Vita seem more attractive and draws in more people, that could make the situation a little bit healthier.
I don't disagree that Sony needs to cultivate or help promote replacement IPs.
In terms of promoting, they're definitely doing so with Toukiden/GE2. I don't see them slacking behind in giving major pushes to the alternatives.
However, in terms of cultivating replacement IPs, imo, this is a 'damn-if-you-do, damned-if-you-don't' situation for Sony.
The reality is that Sony has historically build its franchises and IPs, from top-down, from console with a trickle-down effect with spin-offs and other stuff appearing in handhelds. And it works best this way because Sony's console business and development resources are way, way, way bigger than their handheld business and resources too.
So realistically, if Sony wants to make a replacement IP for FF, etc... they cannot
start those IPs from the Vita. They have to build them as console first to maximise their potential in the global market.
Soul Sacrifice and Freedom Wars are different because nobody gives a shit about monster hunting games on consoles. But it's different for those other franchises, which are primarily console IPs.