In my life, I have only reported two different vulnerabilities to two different vendors.
One of them didn't care at all. They were transmitting usernames and passwords in plaintext and I actually showed one of their engineers this, live, in person, and he just shrugged.
The other one told me something to the effect of "yes, this is very serious. We'll look into it right away" and actually fixed it... 3 years later.
Your biggest hurdle isn't reporting vulnerabilities. That's the easy part.
The biggest hurdle is getting someone to care.