plugin dev with vendor packages
You should not do that at all. The entire process to install plugins via composer instead of the old way is to avoid this conflict. Imagine two of your plugins require the same class/library so you'll end up with a class already declared exception. Instead you can insert your requirements into the composer json of your plugin, run the composer update command for your Craft composer json and you are fine.

All dependencies will be loaded into the correct vendor folder.