There is a beamwidth associated with a parabolic antenna's size and operating frequency. The attitude of a spacecraft is periodically corrected to keep earth within the beam's central lobe. It's approximate so corrections aren't required too frequently and become less of an issue as Voyager gets further away from earth's orbit around the sun. Once it's out of propellant it won't be able to correct for torque from the solar wind etc and the high gain antenna's central lobe will drift until its no longer pointing close enough to maintain an acceptable signal to noise ratio. Unless they can maintain contact through the omnidirectional antenna, Voyager will be effectively lost.

I would think the biggest part of being able to transmit that far is the perfect vacuum of space. There is almost nothing, not even air particles between us and the probe.

If you threw a beach ball from the distance of voyager straight to earth it would eventually make it here.

Vacuum is good, but inverse-square law still applies.

There is almost perfect vacuum between Earth and every star in the galaxy, and yet they don't appear nearly as bright as the Sun.

But you can see the Andromeda galaxy, which is several quintillion miles away.

Sure, but the andromeda galaxy doesn’t have a power budget of 20 watts.

Some photons would make it, just not frequently enough to ascertain any information.

Well, orbital mechanics would mess up a straight toss but at least it wouldn't be stopped by air resistance.

Jupiter might have other plans once your beach ball gets close enough.