Gotta agree with Steelfan on this; the training I've been give always mentions not to go for the areas where there's the possibility of body armour to obstruct your attack.

Instead, aim for places where a sentry/ lone enemy would be vulnerable: neck and head, armpit, side of the ribs under the arm... these are the areas where a knife can do the most damage, typically fatal. You don't need a very long knife to do this either.