What I want to do is use the elasticsearch php client in a typo3 extbase controller. Now I created a composer.json and did the composer install, now I have a vendor directory next to my typo3 directories.
But what do I do next? Can I somehow tell typo3 to use the autoload.php or is there another way to load the classes? I'm kinda helpless.
If you want composer support, then you should consider upgrading to TYPO3 7.6 LTS. You can find some basic documentation about composer at the official website.