I'm guessing you already have a way to figure out the vendor. In this case you can just add conditional statement. The Store Message is done by using a CSS background image:
<div id="storeMessage">
<h1>Your Store Message Goes Here!</h1>
</div>
So you can achieve this by adding an inline styling to the storeMessage div as follow:
<?php
$vendor = "one";
if($vendor == "one") {
$image = "Images/boxshots.png";
}else if($vendor == "two") {
$image = "Images/body_bg.png";
}
?>
<div id="storeMessage" style="background-image:url(<?php echo $image; ?>)">
<h1>Your Store Message Goes Here!</h1>
</div>