What do all of the acronyms mean?
A list of all acronyms can be found on the acronym page
What should I spend my bonus rolls on?
See the gear/consumable page
Should I only take bonus rolls from my vault?
This is a complex question, but generally yes. Your highest priority is getting Aman'muso, Warlord's Vengeance and Zuljins Guillotine, so you'll want to bonus roll the second to last boss on heroic as soon as possible.
For some background on how bonus rolls work, once you bonus roll a piece of gear it is removed from your loot pool on subsequent bonus rolls for that spec. Regularly looting a piece of gear or taking a piece from the vault does not remove it from your bonus roll loot pool, meaning you still might bonus roll that piece. This means you should only consider taking one of your target pieces from the vault if it is your last piece that you need from the boss.
What build should I play?
Ultimately, play what you have fun with. The recommended builds are Aldrachi Reaver for single target and either Fel-Scarred for AoE or Aldrachi Reaver for funnel.
How far behind is X build vs Y build?
TLDR - Sim it. I'll put some sims up here as we get a little further into the season comparing frequently asked builds (like mover vs no mover) with some numbers.