This is info on the php in_array() function:
http://php.net/manual/en/function.in-array.php
The warning is saying the 2nd argument (the array your looking in) is the wrong datatype- So some line of code is sending a bad variable to be used in the in_array() function.
I don't know what is entailed in getting "Flashcoms" to work with vBulletin but if there's any custom code look for any code that uses in in_array function and work from there.