There are two causes of limiting magnitude. First is pure
sensitivity, where no matter how dark the sky is, one can't see
stars fainter than about 6.5. If your vision loses acuity and
blurs things out, it may not at first have much effect on
limiting magnitude, especially if you use averted vision where
things are not seen that sharply anyway. Of course, enough
blurring spreads the signal over too many of your retina's rods
and cones causing you to no longer be able to identify the star
due to 'eyenoise'. If you have light pollution either natural
(Moon, twilight) or artificial, things get bad much faster when
you blur things and your eye can no longer see the signals
because of low contrast. I know it's not giving you a good
answer because the situation can be complex. But you can easily
do an experiment yourself. Get drugstore eyeglasses in various
strengths (+1, +1.5 diopters, etc.) and look at a part of the
sky where the magnitudes are well known, e.g. the bowl of the
Little Dipper has stars of magnitude