In vbs, you cannot use the As keyword that you would use in VB. Just get the IBaseField2 object, however you normally would, and assign it to x. x will then act as an IBaseField2 object, and allow you to use any of that object type's methods. You don't actually even have to Dim x, though you can, if you would like to keep track of your variables better.
Dim x 'As IBaseField2 Dim y 'As MultiValue
Set x = theIBaseField2Object Set y = x.FieldMultiValue