In instrumental vaginal delivery:
In case of failed ventouse, forceps may be used. Ref: Textbook of Obstetrics By Dutta, 4th Edition, Pages 397, 398, 613-614
Full cervical dilatation is the only pre-requisite
Forceps are used in all cases of breech delivery
Forceps may be used if ventouse fails
Ventouse can not be used in rotational occipito-transverse/posterior delivery