1.Identify the required filed from 2lis_13_vahdr and VDITM.
2.Create infoobjects in BW for identified fields.
3.Create 2 separate cubes , 1 for Hdr and 2nd for Item datasource.
4.Now u have 3 infoproviders, 1 Hdr, 2 Itm, 3 material.
5.Combine all 3 infoproviders using Multiprovider and do reporting.