My buddy wants his girlfriend to come on a weekend trip with us to Vegas but I refuse to invite her.
Why don't you want your buddy's girlfriend on your weekend in Vegas?
I don't like her that much
If you are inviting your friend to a Vegas trip, then not inviting their so is exclusionary. Make sure to include in everyone in your plans.
I feel like I just can't be nice to certain people.
You can't be nice to your friend by keeping his girlfriend away, he won't have a good time if she can't come around and then the trip may be ruined. Why not just let her come so that everyone can have a good time? You can ignore her.