If you didn't change it during the installation, by default, your Redmine Stack folder is located in
/Applications/redmine-3.3.2-2. Assuming that is your installation directory, you should be able to find
vendor folders under the following paths:
Also, please have a look at our guide: How To Install A Plugin On Redmine?. I think it could be helpful for you.
I guess you are trying to find an older version of Redmine in order to solve the plugin installation problem, am I right? I highly recommend you to keep your applications up to date. If still want an older version, just tell us which version you want and we will give you the download link.
If you have any other question, please don’t hesitate to ask.