Just about every piece of software these days will have some fraction that is based on open source code, so every software company is to some degree an open source vendor. And in the process of developing this code, the company's developers will need to participate in the community of those projects, and possibly fix bugs and contribute features.
So, whether you are an open source vendor, or for that matter, open core vendor or free software vendor is a question of degree. That means that everyone gets to argue what level the bar is set, and it all descends into silliness.
It reminds me of the fact that, especially at this time of year, everyone in the United States gets to be Irish.
In contrast, there is a very clear definition for open source software. The beauty of it is that it doesn't matter whether the software is written by an employee of open source vendor, an anarchist student, or Microsoft-loving independent consultant. If it has an open source license, it's open source software. As simple as that.