One of the hardest things to juggle when you're running a company is how to deal with your vendors. As a startup, you're generally dependent on single-source vendors. Generally, it's good to make sure that you don't become dependent on one, but oftentimes, the switching costs outweigh the benefit of getting out of the relationship. That means it's critical to pick the right vendor to start with.
We've had issues with cost overruns, delays, lack of communication, incompetence, and just plain old nonperformance. And this is despite the fact that we used referrals for almost every single vendor. Frankly, the only vendor we were happy with was a blind RFP we sent for outbound telemarketing through BuyerZone. In that instance, we went with the first respondent.
It was clear that 1) they knew their stuff, and 2) they wanted our business. Don't underestimate a hungry, smaller firm when looking at vendors. Because they're smaller, they can be more nimble and responsive when you want things done. Although they may see new things and make mistakes, they're generally aligned with you to make things right and offer concessions. Larger vendors may have "seen it all," but oftentimes, that institutional knowledge is lost or only comes up when you need to fix things. Generally, to me, the benefits of institutional knowledge should come from a vendor avoiding mistakes and errors in the first place.
I say this to make a point: when in doubt, responsiveness can be used as a general proxy for aptitude. Those who have systems in place to respond quickly and intelligently prove that 1) they know their stuff and 2) if they don't, they'll find out and get back to you when they do. Our [redacted, because he doesn't seem to get that he's on thin ice] seems decent enough, for example, but he only responds when I want to spend money. If there's no commission available, he's all but unresponsive. Random emails of "what can I do for you" aren't just useless; they're counterproductive when I tell you what you can do and you don't follow through all the way. Just saying "I'll look into it" and then not following up unless you're prompted is crap. Thankfully, he's gotten very little money from us in the past.
So, in addition to checking references and reading proposals and interviewing, look at how quickly those things come. If you have to ask twice, assume the vendor doesn't need your business. And if he doesn't need you, then you shouldn't put yourself in a position where you need him.