Can I use both LATCH and the vehicle seat belt at the same time?

No, you should never use both LATCH and the vehicle seat belt at the same time when installing the car seat. Both the LATCH system and vehicle seat belt are equally safe to use, but you must choose one or the other. When using the vehicle seat belt, store the LATCH straps by hooking the LATCH hooks into the holes on the side of the base to remove slack. (Pages 5, 13, 23, 57)