The easiest way is to change the keystrokes you use to start editing an empty line from a to cc. This allows the built-in cindent indentation to correctly adjust the indent when you enter insert mode, which it doesn't do when you specify that you want to append to the existing (blank) line.
Note that this will only work if cindent is actually switched on. ...
This works with the options formatoptions and comments.
Create the file .vim/after/ftplugin/rst.vim and add the following lines:
For comments, the b:- adds the the dash as a comment character that has to be followed by a blank. See :help 'comments' and :help format-comments
For formatoptions, the r ...
As stated in your comment you have cindent and cinoptions set like:
The options cindent should only be set for the file types you want to use it. It is set automatically for the C or CPP file types (assuming you have filetype indent on in your vimrc [or better: filetype plugin indent on]).
Remove the line set ...