An example would be:
If filter_purchase = 0 Or \"\" Then SetDocVar \"filter_purchase\", \"0\" Else SetDocVar \"filter_purchase\", CStr(filter_purchase) End I
This should be the condition you want
If ((filter_purchase = 0) Or (filter_purchase = "")) Then
@agamike, I believe a single = is used for comparison in a vbs if not and not == link here