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Your question is very open-ended and therefore difficult to answer precisely.
The risk of visual loss would depend on the reasons why orbital surgery is being undertaken.
The main concern relates to damage to the optic nerve (the 'cable' that connects the eye to the brain). This can be damaged either during surgery, or as a result of bleeding in the socket following surgery. (Bleeding can usually be detected and addressed in time to prevent visual loss.)
The risk of damage also depends on whether the condition being treated in the first place is close to the optic nerve. For example, orbital surgery to remove a tumour that is ver close to the nerve is more risky for the nerve than an orbital decompression for thyroid disease.
In some cases, where orbital disease is damaging the vision already, surgery can be necessary to save the vision.
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My understanding is that the overall risk should be quite low if there is no orbital disease. In this scneario, the main concern would be a bleed in the orbit soon after surgery - this can be very sight-threatening if left untreated. However, it is usually possible to detect and to relieve orbital pressure if necessary. Blindness is reported as 'rare' in the surgical literature.
Complications can include double-vision if the eyes end up being slight misaligned after surgery. This may require further surgery to correct. Also, if urgery is undertaken for cosmetic reasons, you should keep in mind that there is a chance of 'overcorrection,' resulting in a sunken or hollowed socket appearance. This can look even more unsightly than prominent eyes.
The important thing is to get prompt attention in the first 48h after surgery if there is orbital pain and visual deterioration.
Overcorrection is difficult to fix, because the fat that is removed from the orbit is difficult to replace.
If it is replaced with fluid, the effect is very short lasting.
If it is replaced with fat from elsewhere, the effect is still transient, and it is not always easy to refill uniformly.
There are synthetic fillers but I have no personal experience in using these.
(Basically, the eyeball looks recessed and the sulcus space around it looks hollowed out. It can make a person look older.)
I don't think it is feasible to keep the removed fat. I would expect most orbital surgeons to aim for a slight undercorrection in a cosmetic case.
While the risk of blindness is small, the risk of a cosmetically unacceptable result is higher. Cannot put a number to it because is depends very much on patient expectation.