So if I buy a arcade game for PS4, for example Minecraft do I have to pay for the Vita version seperately or is that free? Sorry I'm asking for a friend and I don't want to give a answer that could be wrong and I'm having trouble finding information on this through Google so thanks in advance guys. 

It depends if that game is Cross buy. Minecraft for example is not cross buy I believe. They currently haven't announced any plans of it being cross buy that I'm aware of.

It is dependent on game. Some games will give you all versions (Vita/PS3/PS4) and others must be bought separately

Thanks guys, really appreciate it!!!!!!

Wel, most of the Sony titles developed for Vita and PS4 along with some indies are under cross-buy which enables buying once allows to use on both devices(even PS3 if that title is available there too). Its a great saving and your friend should get PS+ too which enables him more saving with free games and discounts across PS3, Vita and PS4.

Also there is remote Play with PS4 which enables to play those arcade in Vita flawlessly as its arcade game, where as other games also runs great.

Buy game on PS4
Play game on Vita through remote play
???
Profit