It already does affect vehicles - and fully at that, because C4-type weapons have 0 "projectile" damage and cannot have any more than that. The issue with splash damage in this engine though is that the splash "sphere" has to touch the very middle of an object, instead of any part of an object's hitbox, in order to affect it. However this gets ignored if the projectile directly hit the object (or is a C4 attached to the object, as in this case) and applies full splash damage regardless of distance. So in order for grenades that have stuck to the ground to actually hit most vehicles, the splash radius would need to be increased drastically. And then it'd be OP against infantry, particularly inside any building where the grenadier can put enough distance between himself and the grenade.
As for buildings: splash damage cannot affect buildings, except if it's part of a projectile that directly hit the building (seeing a pattern here yet?) And I'm pretty sure regular C4/projectiles can't be given the same structure radius damage voodoo magic that demo trucks/beacons get. Defenses and auxiliary buildings are technically not "buildings" though, and the splash already affects them in the same way that it does against vehicles, so it can be done against them sometimes - there are a few cases on some maps where defenses are packed tightly enough together that a single grenade can hit two at once, and deal full damage to both due to grenades not having distance falloff for their damage, so look out for those special cases! One that I can remember off the top of my head is on Ridge War where you can hit both the Service Depot and the Turret at the same time at the external outpost if you think about the things I said.