Can Vital Strike be used with Spring Attack? Can Vital Strike be used on a charge?
No. Vital Strike can only be used as part of an attack action, which is a specific kind of standard action. Spring Attack is a special kind of full-round action that includes the ability to make one melee attack, not one attack action. Charging uses similar language and can also not be used in combination with Vital Strike.
Can I use Spring Attack to make an attack from an ally's square?
Let's use a diagram of a 5-foot-wide hallway to help visualize this question. Periods are open squares. A is the acting character. Numbers are allies of A. X is the monster.
. . . . . A 1 2 3 X
Using the diagram as a model, the question is, "Can I use Spring Attack to move from A to 3, make an attack on the monster from 3's square, then move back to A and end my turn?" The answer is "yes." The key to understanding this is the general rule, "you cannot end your movement in an occupied square." Spring Attack is a full-round action; it is not a move action, then an attack, and then another move action, it's one continuous movement with an attack happening in the middle. Thus, with Spring Attack you're not ending your movement until you end your movement for the turn. To look at it another way, if the character just wanted to move from A to 3 and back to A, that would be a legal move because he's not ending his movement in 3, he's ending his movement back in A. With Spring Attack, the character still isn't ending his movement in 3; the feat gives him the ability to perform an attack as part of the continuous movement from A to 3 to A (regardless of whether that attack happens when he's in an occupied square). (Even with speed 15, a character moving 15 feet from A to 3 and then 15 feet from 3 to A isn't ending his movement in 3... using a move action to move 15 feet and a standard action to move 15 feet doesn't mean he's actually pausing halfway through his movement to change actions.)