I have a complete character bone width boneName_L and boneName_R and a half mesh width Mirror modifier, I setup rigging only for _L side, but when testing the rigging result the other side is moving too, would be good a Flip L/R name button for Mirror modifier for testing separately the other side and after apply the modifier this flipped vertex group name became real…but until… Can i flipped the duplicated half mesh vertex group name from _L to _R like for bones?
That would definitely be a nice feature .
But for now you can use the “mirror bone weight” python script from Thomas “Kakapo” Oppl (huge thanks to him for this script , which is a BIG time saver).
I don’t remenber where i downloaded it , so here it is :
http://mickael.lebihan.club.fr/divers/mirror_bone_weights.py
Note : vertex group name must ended with “.L” or “.R”
Peace :spin:
thanks, 
i found this script in edit mode in Mesh - Script menu and it works.