putting the decoupling capacitor merely/only near the vcc line
There is no difference between VCC and GND, both provide a voltage rail, and they are both defined only by the difference in voltage between them.

When you decouple VCC, the IC cannot pull the VCC pin down with a hard transient.

When you decouple GND, the IC cannot pull the GND pin up with a hard transient.

Either way would reduce the difference between VCC and GND.

An alternative view is to ignore the DC voltage, and treat the IC as the source of high-frequency noise that you want to stop from reaching the other components or the power supply. Having a capacitor on a short path between the supply pins shorts the high-frequency noise (remember that capacitors pass high frequency components and block lower frequencies), however the inductance of the traces to the capacitors form a low pass filter, and together with the capacitor, you get a band pass. Ideally you want a fairly wide band here, so you want high capacitance and low inductance on this path.