My girlfriend refuses to go on vacation with me so I am going to tell her that she's miserable.
That sounds mean. Why would you say that to your girlfriend?
Because I want her to be more happy around me. She is depressing most of the time.
Instead of putting down your girlfriend even further, you should ask her what she would like to do. Maybe she is upset because you don't consider her feelings.
And will this get me a vacation?
It might if she feels better about the relationship and she feels happier with you because you care about her feelings. It will not happen if you tell her they are miserable.