Well, you'd have to convert a string of number to an array then use it in an in_array(). But reading what you psted above, it wasn't working using the in_array() condition so there's got to be some other problem with your condition. If you use the plugin code and this condition do you see ads?
Code:
<if condition="(($post[postcount] % $vboptions[maxposts] == 1)) AND ($show['member']) AND 1">