Preferably, find an old, worn out broom that needs to be thrown out or replaced. Use the hacksaw to cut off the end. Estimate about how long of a handle you will need to reach the vent hooks, then cut the end of the broomstick to be about six inches longer, just in case.
When you've finished cutting your broom handle, screw the small metal cup hook into the freshly cut end. If your cup hook needs a pre-drilled hole, use an electric drill with a small bit.
Once the cup hook is firmly in place, use your new tool to reach the HVAC vent switches. Opening and closing them should be very easy. Keep your vent switch handle in a nearby closet so that you'll always know where it is when you need it.