How do you find the right tech vendor?

The outsource market right now is just flooded with East European and Indian companies looking for projects, which makes it easy to find an affordable vendor. However, if you want a good reputation as a tech advisor or as a CTO, you need to find a vendor that isn't the cheapest but the best in terms of quality.
To me, this consists of many factors with some of the key ones being:

1. Do they have experience with this type of project?
2. Do they offer a flexible team with a possibility of expansion?
3. Is their collaboration model suitable for your style of work?

But you might be asking "Why even use a vendor, though? Surely an in-house team can do it all!" While that may often be the case, I believe that having an external array of professionals can save time and money in development as well as get you out of situations where the current team just can't handle the workload or can't solve a particular problem. There's a bit more discussion on this question here: https://sysgears.com/articles/how-to-provide-your-client-with-the-perfect-tech-vendor/

For now, though, I want to hear your thoughts. What do you look for first and foremost in a tech vendor?