With Greece teetering on the brink of a financial collapse, Monte Mathews decided to postpone his August trip to Athens. Eugene Emmer didn’t. Their different responses to the unfolding economic drama underscore the strong ambivalence Americans feel about the Greek crisis. They’re hopeful the turmoil won’t touch their vacations but fearful it might. Even a whisper of bad news seems enough to send some tourists scrambling to change their plans. The Greek economy isn’t a new story, of course. But it has taken an unexpected turn in recent weeks, as a showdown with the rest of Europe over austerity forced banks to close for weeks. In the short term, visitors worry that their credit cards won’t...