The chassis_vlo has been a component of GTA vehicle DFFs since GTA3. It is the LOD for vehicles, with all components recreated as one model, at a much lower resolution (For Example: The wheels on vlo models are square). The vlo version of a vehicle is controlled like all other LOD objects through draw distance.
The vlo version of models can be particularly useful when creating collision data for vehicles. Their low-detail attributes are perfect for the lo-polygon count
requirements for collision.
This is a high-definition car, with your chassis:
A poor quality San Andreas' car (infernus):