Hello Cesar,
This requirement by way os standard feature is I do not think available in C4C.
However there are ways to accomplish this.
1. Share the access of Manager A with Manager B for the duration of the absence of Manager A.
2. Restrict the Business Role/Access to Manager B is Manager B is not supposed to see all what Manager A gets to see & work with using the Access Right Management in C4C.
3. Let Manager B use Manager A's access for the duration needed and when Manager A is back, she changes her password & continues with her activities on C4C, in which case, Manager A's access have to be revised to what she originally had so that she can continue working on C4C as she used to before going on vacation.
Without an additional license, setting up additional user is not doable, additionally, it will not make much sense to buy license even if you plan to for a temporary user that too only for 15 days.
The Administrator of your C4C solution can easily set up what I mentioned in above points.
Hope this helps!
BR
Ankur