Your only "hard hitting" criteria seems to be LIFNR = P_CNUMBER, you should not "hide" this in the ON-condition, but rather move it to the WHERE-condition.
Please check if you can incorporate index table VAKPA in your join for quick access to sales documents via partner numbers. The new index you created for VBPA might not be needed.
Also see SAP note 185530 with performance tips for SD customer developments.
Thomas