Proportion of cortex dedicated for vision and hearing
20-30% vision. See Sheth &amp; Young (2016) which cites Van Essen (2003).
27% vision, 8% auditory (Van Essen, 2003)
I have seen some earlier references that say 50%. I think this derives from two factors. First, it is 50% in Macaque (see above ref 1).  Second, my feeling (I haven't dug into this in enough detail to conclusively state it, so please don't quote me on this!) is that earlier studies on humans were more speculative and tended to follow that 50% as a best guess in absence of direct measurements. 
As some of the above answers point out, one needs to be careful in defining what part of the brain and what vision means.  If one were to include the subcortical structures (i.e., amygdala, superior colliculus, pulvina, LGN) the estimate of total brain (not just the cerebrum cortex) would be different. 



Sheth, B. R., &amp; Young, R. (2016). Two visual pathways in primates based on sampling of space: exploitation and exploration of visual information. Frontiers in integrative neuroscience, 10, 37.
Van Essen, D. C. (2003). Organization of visual areas in macaque and human cerebral cortex. The visual neurosciences, 1, 507-521.