VehicleFix fixes a bug/issue with entities that you can ride that has existed since rideable entities were first introduced. These entities are known as "vehicles" internally, hence the name. The bug in question is that they don't take into account the rider's own hitbox when deciding if they can go past a block, leading to situations where players and entities suffocate when the game shouldn't have even let them go past those blocks.
So what I did was make it so the game will now take into account the height of a rider (although not the width, due to the sheer amount of extra jank that would need) when determining the collision of a vehicle. This works with modded vehicles as well!