Return path on a PCB
When TX switches low-to-high, the current flows like this:

Power supply Vcc -> PCB Vcc plane -> U1.Vcc pin -> U1.TX pin -> U2.RX pin -> U2.Gnd pin -> "return path" -> PCB Gnd plane -> Power supply Gnd

It's great that you understand that what we call the "return path" will be the nearest plane (in this case the Vcc plane). This makes sense as the fields can't read, so they will form between the metal parts in your PCB no matter what you name them.

In the static DC case, the "return path" will actually be the Gnd plane as that will have the lowest impedance. At higher frequencies, the fields will form to the Vcc plane and the current density will be high in the Vcc plane right under the trace.

So how does the current get from the Vcc plane and back to the Gnd plane for the higher frequencies? 

Well, remember that the impedance between those two planes is fairly low at these higher frequencies. Actually we want to make the impedance between Vcc and Gnd low across the entire relevant frequency range as well (use something like PDNTOOL.COM to design that), so that is no big surprise (hopefully). 

PDN design is well covered in Eric Bogatins book as well.

Let me know if this helped you?