If he used campaign funds for his vacation in Greece, then yes. If he used personal funds, it's obviously not an ethical issue, though it does lead to questions about his judgement if he's taking a vacation while he's supposed to be running for president.
So you think that people running for president should never take a break to relax and unwind? Maybe if Perry had bothered to take some breaks he might not have sleepwalked through his early debates and been able to remember more than two things at a time. Now if Newt were to take a vacation now with Iowa less than a month away, I'd agree with you, but taking a vacation in June was actually a good idea since it was likely the last chance he would have to take one before the campaign season ramped up. Also, if it was to be a real vacation instead of a working vacation it needed to be overseas, and I can't think of an overseas location that would have better politically, and a lot that could have been worse.