Comment 12 for bug 1375346

Vala has preprocessor https://wiki.gnome.org/Projects/Vala/Manual/Preprocessor , I've never used it myself. It might be possible to keep both api versions in the file.

Another option is to submit all the backward compatible changes to repository and keep the rest in a branch or even just as a patch in this ticket. All bleeding edge distros will be needed to patch their packages. Later, when you ready to move to vte-2.91 you'll submit these changes.