Decoupling Capacitor - Multiple Vss pins
If you are using something inferior to a PCB like a breadboard then the decoupler(s) should go across pins 19 and 20 with pin 8 connected to pin 19 using a short wire.

If using a single sided PCB then ditto what I said about the breadboard.

Anything with two layers or more use a ground plane and have the cap(s) close to pin 20 with its earth-end via'd thru to the ground plane.