During import of any mib in MIB Tools, Spectrum finds the common OID between the MODULE-IDENTITY and the Root Node of the mib. If the common OID has vendor, it places the MIB in the vendor folder.
The Root Node of the IB-SMI-MIB.mib is "enterprises" (184.108.40.206.4.1), children of the root node are "infoblox (220.127.116.11.4.1.7779), ibDNSServ ( 18.104.22.168.4.1.32) and ibDHCPServ (22.214.171.124.4.1.9999)". The common OID in between all children of the root node is 126.96.36.199.4.1. As vendor is empty in the common OID, Spectrum placed the IB-SMI-MIB.mib in "Unknown" folder.
As multiple vendors defined in the IB-SMI-MIB.mib, the common OID is "188.8.131.52.4.1"
184.108.40.206.4.1.7779 ( 7779 - infoblox )
220.127.116.11.4.1.32 ( 32 - Santa Cruz Operation )
18.104.22.168.4.1.9999 ( 9999 - Zerna, Koepper & Partner )