C1 what does "la" substitute for here

Regarding your question what la stands for in the idiomatic phrase se la couler douce, I used Google and found the answer here:

 https://www.linternaute.fr/expression/langue-francaise/20419/se-la-couler-douce/ 

Your hunch that la replaces la vie seems to be correct.

Hi Meriel,

The term is "se la couler douce"  --> take it easy.

https://www.wordreference.com/fren/se%20la%20couler%20douce

So "c'est me la couler douce" follows this pattern as above.

Bonne journée,

Jim

Many  thanks!