PS4 -> router (via wifi) -> Vita (via wifi) will never be better than PS4 to Vita as long as the PS4 and Vita are closer together than the PS4 and router, and there is no obstructions present with thee PS4 to Vita that are not present between the router signal.
You are adding an extra step for the wireless to information through when using a router as opposed to ad-hoc. The router method is only best once you get out of the effective wifi range of the Vita and PS4, and then your router is acting as a signal booster. Unless Sony has awful code for the ad-hoc connection, it would not make sense for a connection via router to provide a better signal than direct (given that the PS4 and Vita are closer than the PS4 and router).
I have tried both and the direct connection is night and day better, until I move the Vita to a location with several walls in between the PS4. The PS4 or Vital will be the weakest link if you have a good router, and the performance will be based on their wifi capabilities, instead. However, with he router, you are just adding an additional piece of hardware to slow down the signal as it must then be processed and redirected.