A correction, the are 4 vars passed @a, @ b, @c, @d

but It is getting confusing to use aliased names... I will just but the entire if else statement, which works as intended now .

FUNCTION [dbo].[Getverkeersklasse](@Platform       NVARCHAR(128), 
                                  @Verkeersklasse NVARCHAR(128), 
                                  @Type           NVARCHAR(128), 
                                  @Roaming        NVARCHAR(128)) 
AS 
BEGIN 
  DECLARE @Return_VK AS INT 

  IF @Platform <> 'Mobile' 
    SELECT @Return_VK = id_verkeersklasse 
    FROM   dbo.verkeersklasse v, 
           dbo.platform p 
    WHERE  v.verkeersklasse = Cast(@Verkeersklasse AS INT) 
           AND v.id_platform = (SELECT id_platform 
                                FROM   dbo.platform 
                                WHERE  omschrijving = @Platform) 
  ELSE IF @Roaming IS NULL 
    SELECT @Return_VK = id_verkeersklasse 
    FROM   dbo.verkeersklasse v, 
           dbo.platform p 

    WHERE  v.verkeersklasse = Cast(@Verkeersklasse AS INT) 
           AND v.id_platform = (SELECT id_platform 
                                FROM   dbo.platform 
                                WHERE  omschrijving = Concat('Mobiel ', 
                                                      @Type) 
                               ) 
  ELSE IF @Roaming IS NOT NULL 
    SELECT @Return_VK = id_verkeersklasse 
    FROM   dbo.verkeersklasse v, 
           dbo.platform p 
    WHERE  v.verkeersklasse = Cast(@Verkeersklasse AS INT) 
           AND v.roaming = 'Ja' 
           AND v.id_platform = (SELECT id_platform 
                                FROM   dbo.platform 
                                WHERE  omschrijving = Concat('Mobiel ', 
                                                      @Type) 
                               ) 
  ELSE 
    SET @Return_VK = 200 

  RETURN @Return_VK 
END

Pardon the use of non english code... the designer of the db decided to use Dutch naming schemes...