last playbook with debian stretch

[Regimbal]

Here is my situation:

• I run Ansible and Vagrant on a Mac OSX
• if I run the playbook with Vagrant deploying a debian Buster, everything runs fine.
• When using a debian stretch, vagrant stops at parsing the playbook, finding an error in the letsencrypt main task.

From what I've observed, this coud come from the Ansible version installed on the Vagrant guest machine.
I tried checking out the repo to a commit where Debian Stretch was the main distribution (in such release vagrant file installs Ansible with pip)
There, Vagrant stops claiming that pip is not installed.

Is there any way to correct this please?