Presumably the idea was that since several plugins use the VTE library, eventually in the future the other plugins' code/checks/compat could be factored out into the common utils library.
Correct. I have put some Gtk and Vte compatibility stuff in the common utils lib as there was no attempt for shared compatibility code yet. Also, regarding Vte I wanted to prevent that the utils lib always depends on Vte even if no plugin is enabled which uses it. That's why there is this "cross-reference" in the makefiles. I didn't know a better way to do it. If there is a cleaner way for archiving this then I am open for suggestions.
Vte compatibility code might not be used often but there are definitely some plugins which still need to be ported to Gtk3. So I have made a first attempt to introduce some code in PR #750. There was a discussion about it some time ago but not much feedback IIRC. The bottom line is: this is a first attempt - suggestions/feedback/PRs are welcome. It won't be to much work adjusting the scope plugin to changes of the utils lib I think.