[Dismounted]

In the first sample, you are overloading the function (look it up). In the second sample, you are merely making the second value a string.

See this example (note the quotes):

PHP Code:

// this is effectively what you are doing
is_member_of($vbulletin->userinfo, '5, 6, 10'); 


You need to explode the string and just provide the array to the function. The function can handle an array, or overloading.