Hi @Trout4057,
Welcome to the community.!
Do you get the vacation start date feed from your HR source.? If so, you can create a transform that updates the Identity attribute as (Vacation - Y/N) based on the start date. Here is a sample you may want to refer.
Then create a WF with Identity attributes changed trigger detecting the Y/N changes. IDN workflow has a slack message action available in case you have the slack application installed. See details
If not, you will need to have an HTTP action, calling the slack API’s to send the required messages.