Thanks for the replies everyone!
Upon my first reading of the 6th edition BRB, I came to the same conclusion presented by DeathKlokk. However, after re-reading, I interpreted the text differently. Allow me to explain (NOTE: I'll be directly quoting the BRB here in some cases for clarity. If this is not permitted on this forum, please let me know and I'll be happy to paraphrase):
Under the section "Vehicles and Cover - Obscured Targets" on page 74 of the BRB:
"The difference from the way cover works for other models [non-vehicles] is represented by the following exceptions to the normal rules for cover:"
Before I get into the subsequent bullet points of that section, the wording in that sentence prompted me to read the general rules for cover as they are presented under "Determining Cover Saves" on page 18 of the BRB:
"If, when you come to allocate a Wound, the target model's body (as defined on page 8) is at least 25% obscured from the point of view of at least one firer, Wounds allocated to that model receive a cover save."
OK. With that in mind, back to page 74 for that first bullet point:
"At least 25% of the facing of the vehicle that is being targeted (its front, side or rear) needs to be hidden by intervening terrain or models from the point of view of the firer for the vehicle to be in cover. If this is the
case, the vehicle is obscured (or 'hull down'). If a unit is firing at a vehicle, the vehicle is obscured only if it is 25% hidden from the majority of the firing models that are able to damage the vehicle. If a unit has firing models in
two or more different facings of a target vehicle, work out whether or not the vehicle is obscured separately for each facing, using only models firing at that facing."
So let's break that down. Obviously wounds cannot be allocated to vehicles, so that part can be discarded. Vehicles do not have "bodies" either (as defined on page 8 of the BRB); to me, this first sentence is simply stating that you replace "body" with the targeted facing of the vehicle. Let's move on to the second sentence.
The second sentence states the vehicle is obscured only if it is 25% hidden from the majority
of the firing models that are able to damage the vehicle. This is an exception from the general rule in that normally, a model needs to be merely "25% obscured from the point of view of at least one firer
". So in essence, this sentence is simply explaining that vehicles aren't obscured by being hidden from the view of only one firer (unless that firer was the only model in the unit that could damage the vehicle).
With that first bullet point out of the way, there are a number of situations in which a model does not need to be 25% hidden to receive a cover save. As Magpie pointed out, one of those situations is being inside of area terrain, which is covered on page 91:
"Models in area terrain receive a 5+ cover save, regardless of whether or not they are 25% obscured."
Great! Except that as DeathKlokk stated, the second bullet point under "Vehicles and Cover - Obscured Targets" on page 75 makes a specific exception to this:
"Vehicles are not obscured simply for being inside area terrain. The 25% rule given above takes precedence."
So area terrain (unless it also obscures at least 25% of the vehicle) is out. But what about other situations where 25% obscurity is not required, such as shooting through a 3rd unit (page 18) or shooting through woods (page 102)?
While I'm uncertain on those topics, I can make interpretations.
When it comes to intervening models, normally, a model need only be "partially hidden from the firer's view by models from a third unit" to receive a cover save. In fact, even "if a model fires through the gaps between models in an intervening unit, the target is in cover, even if it is completely visible to the firer." However, going back to that first bullet point about vehicle cover, it is stated that "25% of the facing of the vehicle that is being targeted (its front, side or rear) needs to be hidden by intervening
terrain or models
from the point of view of the firer for the vehicle to be in cover;" this situation seems to be explicitly covered.
That leaves forests, which I'm still unsure about. Here's the text from page 102:
"Models within a forest's boundary benefit from a 5+ cover save. Similarly, if one or more models in a shooting unit have to trace their line of sight through a forest (because they're shooting at a model beyond the forest) then the model being shot benefits from a 5+ cover save. This applies even if during the game you rearranged a forest's trees around a unit - you just need to remember where the boundaries of the forest are."
I would interpret this that if a firer's line of sight goes through a forest at all (whether the firer is in the forest or outside of it) its target receives a cover save; vehicle or not.
While an exception for models near the border of the forest would make sense (as it was in 5th edition), I can find no such rule in the 6th edition BRB. Forests seem to be something of a uniquely double-edged piece of terrain in 6th edition.
Please let me know what you think of my interpretations regarding these rules. Also, if anyone can think of other situations I may have missed that provide a model a cover save regardless of 25% obscurity, throw them out there