If you leave Vodka out in the sun as long as it's closed and not open nothing should really happen to the Vodka other than the Vodka becoming hot.
The Vodka may get hot enough and change taste but it will still be safe to drink even after it's been hot.
If you leave Vodka out in a hot car and the temperature in the hot car reaches 120 F or above then a chemical reaction in the Vodka could occur and then the change of taste would happen.
Even when you cool down the Vodka it still may not taste like it should.
However the Vodka would still remain safe to drink even after it's been hot.
But it's recommended to keep the Vodka out of the sunlight and stored in a cool dark place.