Pipenv has pyenv integration. It just doesn't work with python 3.6 on my machine. My guess is that it tries running pipenv itself with the old interpreter and is no longer compatible. In any case, faffing around with 25 different third-party solutions is a horrible developer experience.
Fwiw what I've settled on that seems the most stable is to use pyenv as the "outer layer", as it shims the python interpreter along with any venvs so you don't get conflicts / heisen-incompatibility. Lets you use anaconda envs as children as well. The real trick, I think, is to pick one system as the canonical master env controller and don't deviate.