Learn Dutch: Dit koude sap drinkt vlot weg, toch? - This cold juice goes down quickly, doesn't it?

This is the facilitive middle voice! Just like you can say "this book reads easily" in English, Dutch uses active verbs to describe how an object undergoes an action. You need an adverb of manner (here, vlot) to make it work. It means the juice "goes down" easily.

What does weg do in this sentence?

It is part of the separable verb wegdrinken (to drink away). In these middle voice constructions, adding weg to the verb (like weglezen or wegkijken) emphasizes that the thing is smoothly consumed, finished, or absorbed.

Sap is a het-word. Why does the adjective get an -e ending in koude sap?

It is true that het-words sometimes drop the -e ending on the adjective, but that only happens when they follow een, geen, or no article at all (for example, koud sap). Because we are using the specific demonstrative dit (this), the adjective takes the standard -e ending.

Could I use snel instead of vlot here?

Yes, snel (fast) or makkelijk (easily) would grammatically work as the adverb. However, vlot is very natural here because it specifically carries the nuance of "smoothly and without hesitation," which perfectly matches the idea of a refreshing drink going down easily.