It is possible that your boss is not interested in discussing your vacation or may be busy with work-related matters. However, if you feel that your boss is intentionally changing the topic to avoid discussing your vacation, you may want to consider addressing the issue with them in a professional and respectful manner. You could say something like, "I noticed that whenever I bring up the topic of my vacation, you tend to change the subject. Is there a reason for that?" This may help you gain more clarity on the situation and allow you to communicate effectively with your boss.