Sharepoint online Vacation Approval flow & Calendar overview

There are many ways to skin this cat, but nothing out of the box and easy. Most are going to require some custom extensions etc. more than likely to achieve something like this in SharePoint. If your requirement could be that managers can see everyone time calendar entry then you could use the built in feature to limit list items so that people can only read their own, then list owners can see everyones that would be easy to accomplish.

However, if you have to have managers only see their employee's then your looking at something custom to trim the views or something that can set permissions on every submission at time of submission. Then you have to think about thinks such as when a manager leave your organization and hire a new one, you have to have a way to update all of those permissions etc. So permissions can get messy very quickly, and that's why having some custom maybe even a fancy powerapp that builds views based on roles instead of assigning direct permissions you could go that route, but with Powerapps, you can really only hide stuff with security by obscurity so you run that risk there.