I'm going to keep my boyfriend from going on vacation
What is your reason for not wanting your boyfriend to go on vacation?
I want him all to myself, not sharing him with other people.
You are behaving in a selfish way by wanting to keep your boyfriend all to yourself. You are controlling and that behavior will always backfire. It is not your decision if your boyfriend takes a vacation, he has a right to do what he wants to do.
I feel like it is not fair he goes on vacation and he don't come back and I'm being left by myself.
Trying to prevent a partner from going on vacation is not very fair. While it does make sense that you are going to miss him, trying to prevent him from going for that reason is not realistic.