Before project planning

How can I simulate the vacation to this team ressource. I can't add real work allocation in advance because I don't know who will be assigned to each Activities?

2 solutions :
- if everyone in the team take vacations on same period (does not seem to be your case), define a specific calendar and assign it to this team resource.
- register real work in advance : no matter if you don't know who will take holidays and when, it is just a planification, and concerns the abstract resource, not the real ones.

What is the good practice to re-assign each activities to the real ressources. In particular for work allocation?

No real "good practice", it is as you feel it.
1) I would recommend to create new assignments and dispatch assigned / left work to each resource.
2) But you can also add assignments with no assigned work, and only dispatch left work to the resources, leaving assigned work the the abstract resource.
In solution 1), you can then delete assignment to abstract resource, in solution 2) you must keep it
In solution 1), you have information on Resource performance, comparing planned to assigned.
Solution 2) can be quicker to manipulate (using Real Work allocation screen for instance to update left work)

I recalcule the planning every weeks, is it good?

Very good practice !
Even better if you check that resource have entered real work for the week, and re-evaluated left work.

How can I store the "original" planning to get the shifting time?

There is no saving of "original" planning. Maybe on a later version.
To get the shifting, set "validated" values to first planning (it should be, as it is what you commited on) and compare planned to validated.

Each ressources should work only 218 days per year, how can I manage this

?
There is no automatic control that can check this for you.
You must use reports. Maybe "Work - Yearly" could help.

Is it a good practice to add professional travel time to an Administrative project

Yes and no.
All depends on how you enterprises deals woth travel time.
Some requires to set them on a project (the project requiring the travel).
Some set them on "out of project" time, so an administrative project is good.

Is it possible assign a team on an activity ?

No, not possible on actual version.

If not, how to assign a group of people ?

Not possible, only one by one : in fact in most cases each resource working on an activity should have difference assigned work.

Today, we are using virtual users representing the differents teams (with the ETP corresponding to each team), and we assign them to activity. It permit to plan the project.
The problem is that we have to clear the field "travail restant" of the virtual user when we assign the activity to real users or to close the activity, and we have to manage users who doesn't exist. If we could assign a team (taking into account the size of the team for the planning) instead of a user, it could permit to plan project without using virtual users.

If implemented, assigning a Team would alway be an ease to insert : when assigning a Team, each resource of the Team would be assigned automatically.
It would be difficult (and maybe unmanageable) to manage team assignment that would be dynamic.

Request recorded as Ticket #1346