Just like Zapivon, The Zakuson is a russian term which is often used to describe the things you eat between the shots, or "Things you eat with your Vodka".
It is highly recommended to eat carbonated, dry and small meals such as white bread while you are drinking, for the sake of your stomach.
Nevertheless, it is important to eat something with your drinks anyhow.
"Alex, did you bring any Zakuson?"
"Ma amarti lecha kodem? Daber Ivrit!"
(English translation: Yes, since it's important and healthy.)
Zapivon is simply the Russian word for "Things you drink your Vodka with", but is not addressed to the fruity flavours that already come as part of the bottle's ingredients. (i.e.: 40% vodka, 50% water, 10% lemon)
Consequently, Zapivon is solely the things you drink alongside the Vodka.
Drinking Cola (coke) with your Vodka is unlikely to be a good idea.
"Alex, don't forget to bring Zapivon with the Namirov."
"Huh? Vladi ya mitromem daber Ivrit! I'll bring Cola."