Achieving min-width with viewport meta tag

Try this:

<meta id="vp" name="viewport" content="width=device-width, initial-scale=1">
<script>
window.onload = function() {
    if (screen.width < 450) {
        var mvp = document.getElementById('vp');
        mvp.setAttribute('content','user-scalable=no,width=450');
    }
}
</script>

Note that I have swapped the initial-scale=1, as I think you had it the wrong way round. You want initial-scale to be set to 1 when width=device-width, so that the page fits exactly in the window. When you set a specific viewport width, you don't want to set initial-scale to 1 (otherwise the page will start off zoomed in).

So you want to change the viewport tag's width dynamicaly .

Here you go :

<meta id="myViewport" name="viewport" content="width = 380">
<script>
window.onload = function () {
    var mvp = document.getElementById('myViewport');
    mvp.setAttribute('content','width=580');
}
</script> 

See:http://www.quirksmode.org/mobile/tableViewport.html

use a @media tag and css. It works wonders. Although it does not supply a minimal width to the view port, this is the preferred way to go.

Here is what I do for the viewport:

<meta name="viewport" content="initial-scale=1.0, width=device-width, user-scalable=yes, minimum-scale=1.0, maximum-scale=2.0">

Then I adjust the size for the panel attached to the viewPort:

@media all and (max-width: 1024px) {
    /*styles for narrow desktop browsers and iPad landscape */
       .myContentPanel{
         width: 450;
    }    
}


@media all and (max-width: 320px) {
   /*styles for iPhone/Android portrait*/
      .myContentPanel {
         width: 320;
    }

}

Obviously you can have intermediate sizes too...

here's more in another example