"That sofa is old but comfortable."
Translation:Tuo sohva on vanha mutta mukava.
It is not correct here. But in other contexts, "vaan" can be used as a translation for "but". It basically means the same thing as "sondern" from German, which is basically the same as "but rather" from English. Here is an example of correct usage of "vaan":
Tuo sohva ei ole vanha, vaan uusi = That sofa is not old, but (rather) new
So it's used when you offer contradictory information to something that you just negated.
Words tend to have more than one meaning, and which one of those meanings they have tends to depend on the context. To add another layer of complexity, the same also applies to English. For example, "pretty" can mean "good-looking", but as an adverb it can mean the same thing as words like "quite" and "rather". The Finnish equivalent of "pretty" (the adjective), which is "kaunis", does not have this strange double meaning. Instead it's the "aika" adverb that has a second totally different meaning, which in English is "time". So take translations with a grain of salt. They're just symbols representing concepts and they are not the concepts themselves. The best way to precisely and reliably understand words from a different language is by observing and analysing their usage in a variety of different contexts.