"When Taskmaster blocks or is blocked, change Taskmaster's A and D to match that of any target character die in the field zone until the end of turn."
In the instance Taskmaster has the highest stats, can you pick the same Taskmaster die to "change" your stats to?
There are no restrictions on his "target", so I would say yes.
UC Vibranium Shield:
"Continuous: Resistence-When a character die you control is KO'd, character dice you control get +2D until end of turn"
Since this is continuous, can I choose to use it later or must it be used as soon as my first character is KO'd?
Continuous means after you have rolled it to an Action face, you "use" it by placing it into the Field. Once there, it remains until it is removed. As long as it is in the Field, it will provide its bonus each time one of your character dice is KO'd.
Side note: it doesn't say "While Active", so theoretically it could stack with multiple Vibranium Shield dice
C Winter Soldier:
"When your opponent uses an action die, Winter Soldier gets +1and1 until end of turn"
Does this stack when opponents use more than one action die?
Yes. He gets the bonus for each Action Die your opponent uses. Note that with Continuous Actions like VibShield above, it is only "used" the one time it is moved to the Field, not each time its ability is triggered.