I created some DOIs for citing VFI Toolkit. You are welcome to use VFI Toolkit without citing it, citation is not a requirement of use, but I will be grateful if you do 
To cite VFI toolkit: https://doi.org/10.5281/zenodo.8136790
The citation is essentially a pointer to the github repo for VFI Toolkit.
To cite the ‘Intro to Life-Cycle Models’: https://doi.org/10.5281/zenodo.8136810
To cite the ‘Intro to OLG Models’: https://doi.org/10.5281/zenodo.8136815
You can also cite the paper announcing VFI Toolkit, which is what people have done until now. But that article is getting a little out of date so I figured that being able to cite the toolkit directly would be a nicer alternative.
Some people had cited the intro to life-cycle models, so I figured I should provide a more formal way to do so.
If you want to create a DOI for your own code, check
https://www.r-bloggers.com/2019/09/getting-a-doi-for-your-code/
and
https://docs.github.com/en/repositories/archiving-a-github-repository/referencing-and-citing-content
Zenodo works very smoothly with github repos.