I think you're right to consider this an issue. Couples should want to spend their vacation time together.
However, blended marriages bring issues to the marriage, which is why second marriages have a higher divorce rate. You were already planning to spend your vacation without your wife, so I think that issue needs to be addressed first.
Why wasn't your wife included in your vacation plans? I think that's your bigger issue than where she chose to go with a girlfriend while you were already planning to be apart.
It's very important in blended marriages to make sure you're putting your wife and time with her first.