Is my one row Key of C Hohner Viennese button box a Melodeon?

As Anahata says, it depends on where you are. In Oz, only the single-row button boxes tend to be called melodeons and, even then, many people will think the name refers only to the ones with the 'cotton reel' variably-voiced versions; the others are often regarded as button accordeons. Diatonic button boxes with more than one row are (in Oz) routinely called button accordeons, as are the Continental system chromatic versions.

Cheers, Rowan