I don't think about them too much.
I do both small format printing and screen printing on apparel (among other things), and I have gotten a few customers after they had used one of the online places you mentioned and experienced their low quality and inconsistent results.
In my experience clients like the piece of mind that comes with knowing they can go to you in person if they have an issue. Once they meet you and work with you, provided you give them a good experience, they will be unlikely to return to an online store that is an unknown quantity. They will have comfort in their trust in you and the fact that they have you to hold accountable, and fear that they will be con'd online.
What's more, I also send out all of my small format printing (but not to vistaprint, lol). And most of my clients are aware of this fact. I do let them know, however, that I have a working relationship with these print venders, and I get wholesale pricing, so they aren't losing out.