If you write in this style then there's the possibility that someone else (or yourself in future) will add a second line, like:

        if(x % 2)
          vec^.push_back(x);
          do_something_else();

under the mistaken assumption that the second line will also only run in the x % 2 case. People make mistakes, this particular one can happen, does happen, and has happened many times, and the compiler absolutely will not inform or warn you, because it doesn't know you intended the second line to run in the same if statement as the first (because C++ isn't whitespace sensitive). Is ruling out an entire class of bug really not worth the effort of just typing two more characters?