How dangerous is a burst vessel in the macula in the eye?

Risky. This is the area of central vision which you use to read and recognize fine detail. A burst vessel in this area can block the view by covering this area with blood or can be a vessel feeding this area and bursting disturbs the blood supply so the area can cease to function well. It is therefore dangerous and needs immediate attention by an ophthalmologist or retinal specialist.
See a retina specila. A number of macular diseases can cause this. You must be properly evaluated and treated.
Could be bad. This wouldn't be good for vision. Vessels don't often burst in the macula, but if they did the blood could damage the delicate photoreceptors and can vision loss. We most often see bleeding in the retina in diabetes or uncontrolled hypertension and in people with very poor blood flow to the eye from carotid stenosis. You can also see it in trauma, including crush trauma to the torso.