Collections* and DOpus.Vectors are different things.
(* In the scripting context. File Collections in Opus itself are yet another thing, of course.)
The docs say metadata.tags gives you a collection:string ("collection of strings"), so DOpus.Vector is not relevant here.
Collections don't have empty or count methods, at least not in JScript. (How collections work is up to the scripting language, so those methods might exist in other languages.)
I've posted Snippet: Enumerating files and metadata tags with examples of how to enumerate the tags in both JScript and VBScript.