Magic Missile cannot be 'targeted'

The Rogue's sneak attack...

If a rogue can catch an opponent when he is unable to defend himself effectively from her attack, she can strike a vital spot for extra damage.

...must be aimed at a 'vital' part of the creature. Magic Missile...

The missile strikes unerringly, even if the target is in melee combat, so long as it has less than total cover or total concealment. Specific parts of a creature can't be singled out. Objects are not damaged by the spell.

...cannot be aimed in such a manner. It simply 'hits' the target.

Only once per casting. Disable construct is a touch spell and uses exactly the same "Holding the charge" rules as other touch spells.

The duration stated in the spell refers to the number of assaults the helpless effect lasts. You do only perform one touch attack after casting the spell. Then, either the attack is successful and the spell discharges on the touched target or the attack fails and you hold the charge normally.