Does neoprene shrinks if you throw it in the washing machine? Because the O'neill Gooru vest size M is to small for me but size L is a little bit big.

No. It will stretch a bit over time, but not much.

Are there ways to shrink neoprene?

no again

not all mediums etc are the same from mfgr to mfgr...try a different brands medium

maybe buy a different vest that fits how you want it to??

if u get the medium it probally will fit when u get it wet neoprene stretchs 6-10% when it gets wet. i always say if it zips it fits, so if u can get in the medium it fits.