This guide is for people who submitted their post and it was not sufccessfull. After Back button post content was away.

I must be genious! I found how to recover vbulletin post lost after:

vBulletin Message
Your submission could not be processed because you have logged in since the previous page was loaded.
Please push the back button and reload the previous window.

STORY: You written post long time, or sleeped PC over night and then wanted to submit your post. But above message appears. You hit back button, but your post is away (maybe only blink without chance to copy it)
I searched 15 minutes for chrome cache viewer, chrome buffer viewer and ofcourse for occurence of above message if anyone came with solution. According to my Google 10 minutes research i am only one who came with the written solution of this issue, woohoooo So im now reallly excited to be able to help you save Your time and your post I almost give up, but i still believed there is a solution. And i got an idea.. vbulletin use so called javascripts to handle these submit fields and forms so when i disable javascript, my message which blink and disappear when i do Back button, cans tay there when javascript is forced not to be executed. One importnat thing i learned: Believe it is possible, your chances of success and that thing making happen substantially increases.

HOW TO RECOVER POST CONTENT:
Disable Javascript )
In Chrome i did it in Chrome Settings (chrome://settings/), "Show advanced Settings" link, is on the bottom of the Settings page. Then click "Content settings" button. Then find "Javascript" and select Do not allow any site to run Javascript. Simply disallow javascript in Your Browser. May try open Your site in a new window and refresh it. Hope that this trick will work in most of browsers. But some may not turn off javascript without restart?

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After disabling Javascript, you should be able to do Forward button and then back button again and hope javascript wont hide your cached post text.
The text can be in html format, so you need to select your post content and copy it incto clipboard (Ctrl + C) and switch to html post mode using button a/A or similar.
Remember to copy your post content and enable Javascript again. Then click New thread again so page is refreshed.

If you have large HTML post, use HTML to PHPBB converter:
http://www.garyshood.com/htmltobb/