Need a step by step procedure to add a new Vendor MIB to Libre.
To add a MIB I think you will generally want to add a new os. The steps are in the docs here:
@drunkenungulate is correct. A MIB, by itself, is useless. It is more or less a translation between numbers (OIDs) and names in the SNMP world. A little bit like DNS is to an IP address.
Creating an OS YAML file is the way to tell LibreNMS how to identify your vendor when polling a new device, and what values (OIDs) are interesting for this device.