Choir.us is worth high $xx to reseller, maybe low-mid $xxx to end user. It's a good domain but frankly the .us extension doesn't help it any.
Tornados.us will most likely be an information site about tornados in the US? Not sure, it's a good keyword but I can't think of a good usage for this name.
Vocabulary.us is probably your best domain out of those, an end user could probably make a website for SAT vocabulary or something of the like. To a reseller, probably low $xxx, to an end-user, probably mid-high $xxx.