You could use a condition in the postbit(_legacy) template:
Code:
<if condition="$post[postcount] % $vboptions[maxposts] == 1">
<div style="padding:0px 0px $stylevar[cellpadding]px 0px">
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="0" border="0" width="96%" align="center">
<tr valign="top">
<td clas="alt1">
<div align="center">
ADSENSE CODE
</div>
</tr>
<tr>
</td>
</tr>
</table>
</div>
</if>
top of the postbit template.