Check your Preferences > Display Performance options. If it's set at Typical, by default, vectors will used a lower-res preview. If you change it to High Quality, it will render vectors as good as your screen can display them. You can override this under View > Display Performnace, but you can also set this on an individual basis... e.g select that graphic, right-click and select the Display Performace for that particular image. By default it will use the overall View setting.
But, if you have a decently-powered computer, I recommned you change your main setting as above to High Quality