If you choose just one target for V.A.T.S., that creature will be destroyed when V.A.T.S. resolves as long as it’s still a legal target, regardless of whether or not its toughness has changed since V.A.T.S. was cast.

In the rare case where the legal targets no longer all have equal toughness (probably because of a triggered ability or special action) when V.A.T.S. tries to resolve, it won’t resolve. None of the chosen creatures will be destroyed.

Players still get priority while a spell with split second is on the stack; their options are just limited to mana abilities and certain special actions.

If the resolution of a triggered ability involves casting a spell, that spell can’t be cast if a spell with split second is on the stack.

Players may turn face-down creatures face up while a spell with split second is on the stack.

Split second doesn’t stop triggered abilities from triggering. If one does, its controller puts it on the stack and chooses targets for it, if any. Those abilities will resolve as normal.

After a spell with split second resolves (or otherwise leaves the stack), players may again cast spells and activate abilities before the next object on the stack resolves.

Casting a spell with split second won’t affect spells and abilities that are already on the stack.