There are multiple approaches to your solution, may below hints can help you out
1. If number of vendors your case is less, you can create storage location for each vendor, this will enable to track inventory with vendor, You can capture the numbers or material used in entry sheet long text, but you have to post goods issue for this using movement type 221.
- You can enhance service entry sheet, using user exit -SRVEUSCR User screen on tabstrip of entry sheet or SRVDET User screen on tabstrip of service detail screen
- At time of service entry save, you should be able to create reservation for the materials that consumed using reservation BAPI and then post good issue, Reservation can have required details in header text and item text.
2. You can simply create 941 copying movement type 541, In case you may have to follow multiple steps before you are able to consume this material using 221.