I have been doing some amount of work on combat log analysis recently. There are certain things for which neither WWS nor WoL are suitable.
When Blizzard redid the combat logs in 2.4, they added GUID columns for each participant in the line (source and destination or, if you prefer, subject and object). The GUIDs can be used to uniquely identify a participant.
According to WoWWiki's page on UnitGUID(), you can determine the type of object using a bitmask. If you shift the GUID right 52-bits and do a boolean AND with 0x7:
(In US realms, the third digit from the left will show this. In EU realms, the higher bits seem to be used for something recently.)
So, given the following GUIDs:
Yes, Yogg-Saron is a vehicle. Other mobs which appear as vehicles include the Constrictor Tentacles, Ignis, and Archavon the Stone Watcher. All of these mobs have effects that physically hinder or affect a character in some fashion. This suggests that these are designated as vehicles as a side effect of the internal mechanic for these effects.
There is something hilarious about Yogg-Saron as a vehicle though.