To get the most bang for the buck against an Injury you would use first Surge of Vigor (Say our base 40 now becomes a 80) and then take your 80 and use your Burst of Vigor Savage trait for 160, but only against injury. this could not be used when casting a spell and used as drain. Because that is a Drain test not an injury test (even though if you screw up you could get injured).
Also remember The Savage Trait last only for a single Injury, Surge of Vigor lasts until the End Phase of the round, that is why it must be done before ANY vigor roll is made. The bonus to the Savage Trait is that it can even be used after you roll and see you need a higher result!
In addition: Remember your Base vigor is what you gain from your Endurance Core ability, Background results and Template Vigor. All other talent bonuses or Magic bonuses are added on after any doubling (or even Quadrupling in this case) and the max Vigor you can have is Double your current Endurance score.
Gang up: No you get no bonus to defense because they cannot attempt to Gang-Up.
Your first question was a good one, the second is milking the goat a bit too much. Remember trying to power game too much will have you ending up with unusable or wasted (Sp) talents. Plan out our character and better yet take it to a template that fits with his story or adventures he is in. You may find you like playing a well rounded character or perhaps not, either way here is your official answers.