Vim double-indents python files

This happens because of default vim indentation plugin for Python. It inserts 2 shiftwidth on the first line below [.

You can see code which causes this behaviour here: https://github.com/vim/vim/blob/0b9e4d1224522791c0dbbd45742cbd688be823f3/runtime/indent/python.vim#L74

I would recommend you to install vim-python-pep8-indent plugin which does indentation inside parenthesis exactly as you need.