It's a good idea to set up your system so that it can handle issues being flexible when someone is on vacation or sick leave.
Here are three ways of doing it by using the field User groups, with group permission or by issue rules:
We recommend that you use User Group as an issue field, and use the use the settings where user and user group are dependent.
For incoming issues or emails, create filters that take the user group into consideration. Use these filters in the Issue list or as portlets in the Dashboard.
All users in the relevant user group would then use the filter for their group to see just the issues for their group.
If a single user is absent (Vacation/sick leave/etc), his/her issues will still be shown to the group if something happens, like an email response. The issue can then be handled by another user in the user group.
Another way is to make everyone in a project have access to all new/incoming issues, say in a list of incoming change requests/bugs/etc. That way there should always be someone that can see and handle the issue while the original recipient/handler is absent.
If you want to redirect incoming/updated issues from one person to another, you can use an issue rule for this:
(paraphrasing the issue rule in normal text:) "If the issue is changed or created, and the owner is X (the person who is away), then set the owner to Y (the other, specific person that takes over for X)".
Then, if there is an update on an issue that is associated with the "away" user, the other specified person should get the issue instead.