It is correct, both versions occur, with semicolon and with slash. However, the slash version is the more professional one and is used in specialised literature. So I have come to the conclusion that VOR/DME is the better option, although it is not the most common in OSM. A few years ago, there was often a version with lower case letters (vor-dme), which I converted to upper case.

I hope you can agree with me.