Log in via cookie - when you log in, check the box to "remember me".
If you are inactive for 15 minutes, the board considers you offline. vBulletin3 does not preserve the entered text, which would come out of the blue, since it has forgotten you. When the forum cookie is present, this doesn't happen, regardless of how long you are inactive.
I've long gotten in the habit of writing long replies in a text editor, though part of that is being anal about fixing the typos / grammar / etc. You can often recover lost text if you take your browser offline, then hit the back button.