step-by-step:
Go to "Issues" in Jira Administrator Menu
Go to "Notification Schemes"
Click "Notifications" in Vacation Manager project
Go to "Add" in chosen event (e.g. Issue Resolved)
Select "User Custom Field Value" checkbox and select "Substitute"
Save changes
Go to "Issue Security Schemes" (above "Notification Schemes")
Click "Security levels" in Vacation Manager project
Click "Add" in default Vacation Manager plugin Security Scheme
Select "User Custom Field Value" checkbox and select "Substitute"
Save changes
Note: Following these changes, the person added as a Substitute can see all details on this vacation document - attachments, comments etc.