I guess the hood is fairly old. This tends to happen with vinyl hoods as they age.The cracks nearly always appear where the folds are in the roof.
You could try a repair if you can find any where near to you in France that does vinyl repairs. But, you are likely to see other cracks appear as it ages.
I had 2 hoods (in vinyl) on my old Mk2.5 and then I spent a bit more and had a Mohair hood fitted. Best thing I ever did, lasted ages and still looked excellent after 6 years when I sold the car.
If you can afford it get a Mohair hood…false economy not to in my opinion.
You can kind of waterproof it by sticking some vinyl on the underside; use a contact adhesive, seal the edges with gaffer tape while it sets, then use black silicone to “fill in” the tear. It will serve two purposes; provide some weatherproofing, and act as a constant reminder to buy a new hood. A perfectly decent vinyl hood is about £150, lasts as long as any of the more expensive brands (though BAS hoods do very closely resemble OE).
Your picture shows the classic small splits that occur in the seams above the doors. This is the first stage of vinyl roof degeneration meaning that the material is tired and has become inflexible. Your split is the second stage where one of the seam splits extends to the main panel.
If you repair this and I would say that small split has been caught early enough to effect a good repair as described above, the roof could last another year or two. I would repair it and aim to replace next summer when the weather is warmer - easier to work in and vinyl more workable.
Thx to you for all these answers
Very helpfull, i think i am gonna give á try to slow it down
I understand i Will have to change the hood at one point
For now , i drove 700 miles from England and had 3 Hours of hard rain, and havent seen any leak
So the car is waterproof
Its only a car for sunny days, so it should be ok for a year or two, unless i décidé to change for a blue mohair hood sooner