Another way based on amadan:
SELECT * FROM [Purchasing].[Vendor] WHERE
( (@url IS null OR @url = '' OR @url = 'ALL') and PurchasingWebServiceURL LIKE '%')
or
( @url = 'blank' and PurchasingWebServiceURL = '')
or
(@url = 'fail' and PurchasingWebServiceURL NOT LIKE '%treyresearch%')
or( (@url not in ('fail','blank','','ALL') and @url is not null and
PurchasingWebServiceUrl Like '%'+@ur+'%')
END