Yes, the country is vulnerable for more attacks. Whenever there is an attack of a certain place, it tends to always re-open. However, that re-opening does not mean the place is no longer susceptible to terror attacks. Any place, regardless of past attacks, is vulnerable. Places that re-open tend to be more commonly thought of as future targets.
Ataturk Airport in Istanbul was reopened following a terrorist attack that took place there. However, all can't be quiet on the Western front after this. Given their proximity to Syria, Turkey is vulnerable to the ISIS cells that exist there. As a result, they should prepare themselves for more terrorist activity.
At this point in our lifetimes, all countries are vulnerable. Now, I do not believe that since reopening they will be attacked right away. They are still on alert, but once things settle down, and the workers and civilians get back to "normal", I believe an attack will happen again.
The sad thing is we are all vulnerable to more terrorist attacks. It seems that these days terrorist stake out a place to see what is most vulnerable than attack there. I would like to hope that this airport is taking more security measures and that this country is as well. Only problem and the scariest part of all none of us are safe at 100%.