Having played both, this game is far more lenient with the map stuff, I don't think you'll be missing a small spot if you just explore everything properly. If I remember correctly, there's also an item that increases the range that you can 'chart', so having that equipped is a good idea.
It's not 60 FPS like it is on the PS4, of course. For the most part it runs
okay on the Vita at 30 FPS, with the occasional drops here and there. Some very special places can cause more than the occasional drop, but for the most part it's a very, very impressive game for the Vita. Here's random footage off YouTube from someone playing the english build of the game in English on the Vita:
https://www.youtube.com/watch?v=XeJFry--fLo&t=3m56s
Now, for your second question though, it depends. The PS4 version adds a couple dungeons for Dana, along with a few boss fights. These flesh out her story a bit more, but for the most part it's... well, backstory. It won't really change any of the plot and the Vita version will give you a perfectly fine version of the Ys VIII plot for what it is. Other than the dungeons and the story, PS4 also has different 'attack forms' for Dana, which the Vita version lacks, as well as a new difficulty mode, Inferno. Inferno is harder than Nightmare, the highest you can select on the Vita, and features enemies that hit harder and are sped up. I think these are probably the main differences, so if you think you'd be fine sacrificing the 60 FPS/more backstory for the game on a portable system, go for it.
I posted on another thread the other day, but I bought it in japanese as soon as it came out and the game is a really, really great looking PS Vita title. I had the feelign I was playing something special on that system, which the PS4 version obviously can't replicate as even though it's 60 FPS and boasts high res, at the end of the day is still limited by what Falcom can do.