I am a level 85 fire mage and do a mix between PvP + PvE but not sure what gear to get. Should I get the vicious gladiator set or the firelord's vestment set or something else :P

What is 'World Of Warcraft Fire Mage Gear'?

You can not do a mix of both. If you want to PVE, then run dungeons, raids, etc. If you want to PVP, then do BG's, Arenas, and Rated Battlegrounds. Having a mix of both is useless. What you can do is get dual spec, and get both PVP and PVE gear. So that way when you want to PVP you can equip your PVP gear and change into your PVP spec, and vice versa for PVE.