Really it seems the issue is just that vehicles are not triggering the SPELL_SUMMON or whatever it is that they added so we could link pets to players. They either need to trigger that combat log event too or need a new similar combat log event of their own.
3.1 also finally fixed the src arguments for SPELL_AURA_REFRESH and i've been assured by Zoot that SPELL_AURA_APPLIED_DOSE and _REMOVED_DOSE will have their src arguments fixed in an upcoming PTR patch too.
The main difference between UNIT_DISSIPATES/DIED/DESTROYED is mainly the english text that is displayed for people showing english-like combat logs. In all cases, the unit is considered destroyed and you can end processing data for that unit.
I'm guessing it could be used for units that despawn (say a leashed boss, or adds that despawn on a boss death)?