You only need to worry about vendor MIBs if you’re trying to query/monitor something vendor-specific. Standard stuff like ifInOctets and ifDescr are included in everything. I like to either read through MIBs by hand, or load em in a MIB browser (like mbrowse or use vendor C’s snmp explorer tool) to search/dig into a table/value that interests me, then snmpwalk a base OID to find instances and enter the pattern directly into whatever collector tool I’m using that week. If you’re trying to use vendor MIBs, you also have to deal with dependencies and conflicts, so prepare for some hair-pulling hell.
I’m still on the hunt for a decent F/OSS trap receiver that will allow me to just load up MIBs, and have some simple config/UI method for selecting trigger actions/notifications, maybe some filtering and unknown-trap handling etc. Trying to snag traps with snmptrapd and parsing text output is uncooperative. Inconsistent formatting of key/value etc.