If you already own a Vita, and can afford a PS4, I don't see why it would hurt to get both.
But if you're considering buying a PS4 and a Vita just for remote play, I'd say stick with just the PS4. Don't buy the Vita just as an accessory. Take a look at the games available for it and see how many grab you.
We get a lot of JRPGs, and there's a few blockbuster exclusives like Killzone, WipEout, Uncharted and Gravity Rush. Not to mention a ton of indies like Hotline Miami, Fez & Guacamelee, if that's your thing.
Hope this helps.
PSN: PJB-11 / Now Playing: [PS3] Final Fantasy X / X-2, The Wolf Among Us. [Vita] Sorcery Saga, Dynasty Warriors 8, Velocity Ultra, Dustforce.