Currently, I just open vim in the terminal, write what I need, then copy the text. I open dmenu using a shortcut, type something like "unwr", which is sufficient for selecting the "unwrap-clipboard" script of mine, press enter, which unwraps the text on the clipboard. Finally, I paste it in the email client.
I know it might seem a bit tricky, but it's better than what I did before. I used to set the textwidth to a really large number like 9999, highlight the text I wanted to unwrap, and then type 'gqq' for formatting. And don't forget, you also have to wrap the text back if you don't want to change the original format of the text file you wrote.
You can use soft-wrapping in vim, but I don't prefer it. Soft-wrapping is not as convenient to me as hard-wrapped text when using vim shortcuts. For instance, if you are using soft-wrapping in vim and you press 'o,' the insertion mode will start at the end of the paragraph because vim will consider the entire paragraph as a single line. However, there are many situations where I only want to insert text in the middle of a paragraph. You will also most likely set j to act like gj and k to act like gk, to make the cursor move between lines that are visually separated but actually form a single line. I don't like this either.