Chris,
No prob! My fault. And you gave four answers as well 
.
Certainly clicking on the string of text (obsidian://open?vault=MyFile etc) in a text file works perfectly.
But whether or not I Import my Vault(s) into DEVONthink, or Index them, as Dan suggests, I want to be able to open them from inside DEVONthink - as (Obsidian) files, rather than having always to open Obsidian itself, which seems to know where the root of a vault is.