Hello Lionel,
you don't need a virtual link for each router of area 2 if you have setup a virtual link between router2 and router3 you have provided a point to point logical link to area 0 and this is enough for all routers in area 2, actually it allows area 2 summary routes to reach the backbone area.
To be noted that area 3 in your picture can never be attached to area 0, because a virtual link cannot cross multiple areas to reach area 0.
So if you question is there is a way for area 3 to be part of this OSPF domain the answer is negative.
In real world the use of virtual links is limited to temporary fixes after a network merge and they are quite rare
Hope to help
Giuseppe
