While we replicate a vendor from R/3 system to SRM system it may not be necessary that the Vendor number in both systems are same.
SRM provides three options for Vendor numbering while transferring from R/3.
The options are as follows -

Fig 1 - BBPGETVD screenshot in SRM system
Given a case wherein the R/3 Vendor number is already assigned in SRM to some other object, the vendor number in SRM would be different from one in R/3.
Now this creates lot of confusion while creating SCs, since the requesters are used to R/3 vendor numbers and when they create SCs using them, they invariably uses wrong Vendor number and get error messages.
SAP provides a table in SRM wherein an User can check the corresponding Vendor number in both systems if he/she knows the Vendor number in one system.
Go to SE16, table BBPV_BUPA_VENMAP.

Fig 2 - BBPV_BUPA_VENMAP table view
Here field 'BP number' is the Vendor number in SRM, while field 'Vendor' is the vendor number in R/3 system.
So if a requester knows R/3 vendor number, he needs to key this number in 'vendor' field and on executing he would get corresponding Vendor number in SRM.

Fig 3 - BBPV_BUPA_VENMAP output
The output would have both the Vendor number details, thus requester can create SC with correct data.