Hi Andrew,
Thanks for your answer. We have appreciated it !
Switching the vacation schedule seemed to be a good idea since the tech may change its vacation state itself but in practice this is not usable, of course, and you've right !
Therefore when the Group Manager should execute some task once a week it would be better, when a conflict occurs, that (s)he takes the tech out of the group for that week.and includes it again at week's end.