Interesting question.
Answer is, Vlaar had more time and space to score his long range goal because he was a centre-back bursting forward from the centre of our defence.
Delph, can only burst forward from middle of midfield. Big difference.
Why?
Nobody plans for a centre-back bursting forward. Nobody there to mark him or close him down.
Time and space is something the goal scoring midfielder needs create for themselves because they don't get much of it in the Premier League.
Delph can't score without it. Or with it, if you consider the effort skewed wide from 12 yards against QPR!
But you've already alluded to Delphs dribbling ability which would make him one of the better bets to score from midfield. I've seen time and again that Delph has the ability to go past players which means he's actually pretty good at making space for himself. Oh and as for his miss against QPR, if someone misses a penalty does this mean they will never score one again?