meaning is same but when you slap someone in the cheek, we 'often' describe as someone slap in his 뺨, instead of 볼 and generally when you indicate someone's cheek we 'often' use 볼 I think.

I just checked the dictionary and it says 뺨 is from upper chin to temple and 볼 is middle of 뺨. So 뺨 includes 볼. However, I didn't notice the difference until I check the dictionary. I also don't think most of Koreans will distinguish these words when they use. it is totally interchangeable and only difference is just sound for me.

We usually say "볼" as a part of our body, but "뺨" is often found in proberbs or idioms. \^o^/