There was a time when there were "regular" cell/mobile phones and these new "smart" ones. To describe the latter category the English term got translated directly to älypuhelin. And when cell/mobile phones became kännykkä, some used the coined term älykännykkä.
Quite quickly they became commonplace, so there were no longer a need to underline the (sometimes questionable) smartness and the prefix äly- got dropped. If you happen to have a non-smart cell phone and need to emphasise its non-smartness, you say (ihan) tavallinen kännykkä : (just) a regular, ordinary cell phone.
But all in all the change has been fast. In practice there are no landline phones any longer, almost all phones are cell phones, so there is less and less need for a separate word for cell phones. The word puhelin : telephone is coming back as Steven Jobs prophesied.
Of course I don't know the situation in English speaking countries, but I can well imagine a similar development: when a certain type becomes the almost only one, why use a longer name when the shorter one will do.
In any case report this and wait what the Duolingo team will decide.
Vai is used in an either-or question where you have to make a decision between two items e.g. You only have water and coffee to offer. So you ask your guest: Do you want coffee or water? Otatko/Haluatko kahvia vai vettä? In this case here the answer is open. The person asks, if you have phone or a camera. She doesn't know the answer. You could have one of these items, both or neither. Check this page here: http://randomfinnishlesson.blogspot.com/2012/09/tai-vai.html
I would add that if you are talking about countable things like kamera an kännykkä, vai would require you to use the nominative rather than the partitive.
- Onko teillä kameraa tai kännykkää? Do you have a camera or a cell phone? (either one or both that I could borrow, or that you could use right now)
- Onko teillä kamera tai kännykkä? Do you OWN a camera or a cellphone or both? (in general)
- Onko teillä kamera vai kännykkä? Do you have either a camera or a cell phone? (you either own or use only one in the current situation; the idea of having both has already been excluded as impossible)
so this appeared as a click and select, it had both nominative and partitive forms of Kamera (kameraa) in essence, rather than saying "you've missed an a" it marked it wrong for a single letter off. Harsh duolingo, especially when the rest of the sentance was perfectly correct