This is what you're basically trying to achieve I think but I have no idea how vbulletin gives users the ability to select a fluidity style: I'd really need to look at the cpanel to be honest to see what options are available.
Code:
<table border="0" cellpadding="0" cellspacing="0" align="center" background="yourstretchy background bit.jpg">
<tr>
<td align="left"><img src="http://www.liverpoolway.co.uk/images/leftbit.gif" border="0" /></td>
<td align="left"><img src="http://www.liverpoolway.co.uk/images/rightbit.gif" border="0" /></td>
</tr>
</table>