Your VistaPrint example is not a responsive design. It's exactly the same width when it's in a full widescreen, compared to when you start to narrow the browser (about 956px).

The whole concept is based upon a fluid grid using proportions in place of fixed sizes. So for example, the whole width of your site may be 100% or less, but if it were less, you would have to use other elements to position the site on the screen where you want it.

To expand on that, if your largest screen size was 1920px and you wanted a site size of (say) 1280px, then the site width would have to be set at 66.666666666666%, to present it correctly. Clearly then, unless you created an element of approximately 16.667% to position to the left of the site body, your site would be off centre to the left of the screen.

Furthermore, you have to consider different screen ratios. If you design for widescreen, how's it going to look on a conventional screen, or indeed a mobile?

In addition, Internet Explorer (as always) is way behind the curve and some of the elements essential to get a proper implementation, fail in IE. Therefore, you need to use a javascript backup script to compensate for IE's shortcomings. So then ask, what if Joe Bloggs has got javascript turned off? So now you need to consider how to make the site degrade gracefully.

Responsive designs sound like a great panacea, and if Microsoft ever gets their act together, or people simply banish IE from use, one day it may be. In the meantime, don't run away with the idea it's an easy option; it's not.

It takes a lot of careful consideration, planning, testing and coding to make it work properly over the majority of browsers.