Do you get this error when you post a long post in the vBulletin forum:
The text that you have entered is too long (16356 characters). Please shorten it to 10000 characters long.
If you have admin access then you can fix this issue by changing this setting:
1. Log into Admin CP.
2. Expand Settings.
3. Click on Options.
4. Select Message Posting and Editing Options.
5. Now change the Maximum Characters Per Post value from 10000 to your desire limit.
Now go to the forum and try to post again with the long content and see if the issue is resolved.