Hi there,
Hope you're doing well.
You can just create a Message variable on your Vacation_Detail screen and use it to build the entire message that you pretend to show.
Everytime you execute a validation, if it fails, you concatenate that respective message with the previous ones and so on.

I also created an IsValid boolean variable that will allow you to check if you show the error message or the success message - I believe you want to maintain your success feedback message :)
Your Save screen action should look like this:

Please refer to attached OML file.
Kind regards,
Rui Barradas