VIM textwidth has no effect

Try gggqG to apply the new text width to the whole buffer.

• gg means : go to the beginning of buffer
• gq means : reformat the text included in the motion
• G means : go to the end of the buffer

(It will work if the format options are correctly set, as detailed in Zyx post)

On the other hand, you could also force your existing text files to a width of 72 characters by adding a modeline at the beginning or end of your file. See :help modeline.

Something like vim:tw=72 should work.