The source currently builds and creating a vector layer from a path does succeed. However, nothing is shown in the vector layer. Probably, because of some code I temporarily disabled, or due to other changes in the code since the patch was first made. Most of this was done last August. I got distracted by something else and left it in its current state
Besides patch 1, I think I also integrated some of the follow-up patches (but not patch 2).
I noticed there is considerable duplication between the text layer and vector layer classes. It might make sense to eventually have an intermediate class with common functions, since it's not unlikely we will have other layer types in the future.
A whole lot of changes needed to be made to make it build on master. Several assumptions have been made, parts have been disabled, just to make it compile - Wormnest.
January 2022: There's a large #if 0 part in app/tools/gimpvectortool.c. I think most of that code was already integrated into GIMP in app/display/gimptoolpath.c, but I didn't get around to checking if that is true for all of it.