vBulletin originally let you report your own posts. Then it changed, a few versions ago.
Here's a workaround: Report a post near your post and explain in the Message area which post you are really reporting.
A sneakier trick you can use:
1. Note the post number of your post. For example, you can hover over the quote button for your post and see the post number in the Safari status line message.
2. Click the Report Bad Post icon next to some other post.
3. In the address bar, change the post number at the end of the URL to the post# of your post.
4. Press RETURN to refresh the page with the new post number, before you fill out the message.
5. Fill out the message and click Send Report. It would still be smart to explain in the message that you are reporting your own post.