Finally find out the bug. In vote-my-post.php file line no 605 we can see this code $html = "<div class='" . $class_1 . "'><div class='first'><a href='javascript:void(0)' id='up_count' title='Click to like this post'><img valign='middle' src='" . plugins_url( '/images/upvote.gif', __FILE__ ) . "'/></a><div class='counter' id='vmp_up_counter'>" . $up_count . "</div></div><div class='other'><a href='javascript:void(0)' id='down_count' title='Click to dislike this post'><img valign='middle' src='" . plugins_url( '/images/downvote.gif', __FILE__ ) . "'/></a><div class='counter' id='vmp_down_counter'>" . $down_count . "</div></div> </div><div class='" . $class_2 . "' id='vmp_message'></div><br/><br/><br/> ";
i have just replaced this code to
$html = '<div class="' . $class_1 .'">
<div class="first"><a href="javascript:void(0)" id="up_count" title="Click to like this post"><img valign="middle" src="' . plugins_url( "/images/upvote.gif", __FILE__ ) . '"/></a><div class="counter" id="vmp_up_counter">' . $up_count . '</div></div>
<div class="other"><a href="javascript:void(0)" id="down_count" title="Click to dislike this post"><img valign="middle" src="' . plugins_url( "/images/downvote.gif", __FILE__ ) . '"/></a><div class="counter" id="vmp_down_counter">' . $down_count . '</div></div>
</div>
<div class="' . $class_2 . '" id="vmp_message"></div>
<br/><br/><br/>
';
now it is working fine