How to fix the indentation of an entire file in Vi

Why not use tabs (introduced in Vim 7)? You can switch between tabs with :tabn and :tabp, With :tabe <filepath> you can add a new tab; and with a regular :q or :wq you close a tab. If you map :tabn and :tabp to your F7/F8 keys you can easily switch between files.

If there are not that many files or you don't have Vim 7 you can also split your screen in multiple files: :sp <filepath>. Then you can switch between splitscreens with Ctrl+W and then an arrow key in the direction you want to move (or instead of arrow keys, w for next and W for previous splitscreen)

Use \r instead of \n.

Substituting by \n inserts a null character into the text. To get a newline, use \r. When searching for a newline, you’d still use \n, however. This asymmetry is due to the fact that \n and \r do slightly different things:

\n matches an end of line (newline), whereas \r matches a carriage return. On the other hand, in substitutions \n inserts a null character whereas \r inserts a newline (more precisely, it’s treated as the input CR). Here’s a small, non-interactive example to illustrate this, using the Vim command line feature (in other words, you can copy and paste the following into a terminal to run it). xxd shows a hexdump of the resulting file.

echo bar > test
(echo 'Before:'; xxd test) > output.txt
vim test '+s/b/\n/' '+s/a/\r/' +wq
(echo 'After:'; xxd test) >> output.txt
more output.txt

Before:
0000000: 6261 720a                                bar.
After:
0000000: 000a 720a                                ..r.

In other words, \n has inserted the byte 0x00 into the text; \r has inserted the byte 0x0a.