please someone look into this i dont think it will be too hard to accomplish 
Unfortunately if you were to do this the nub wouldn't fit and you would end up mutilating the exterior case.
the nub on the PSP's use a flat circuit that slides over the contacts making it very easy to fit one in a small space. Not so with the PSVita however.
The Vita uses a six-axis nub that sits atop a setup of rings that meet certain contact points and transmit its position as it rotates in a 180 degree arc, it make it much more accurate and pressure sensitive but it also requires more space then the latter.
Now having said that if you really just wanted the actual thumb stick itself on the vita for just the look you could cut the thumb piece off and shave it down until it sat properly on the PSP but it would still work the same.
If you wanted to achieve just that you could save yourself the trouble and buy attachments that would fit over top or even replace the current PSP nub with a larger pad. I believe they are still fairly easy to come by and should be available on DealExtreme for a reasonable price.
