Fall back for old browsers when using vw, vh

greetings SStory, , to get a "backwards" compatible for older browsers, I do not use any javascript, what I do is define the original CSS spec for and OLD desktop display often using a fixed width as 200px or 7em, then to gear up the CSS response for a Responsive width I advantage the CSS capable newer  browsers by using the   @media screen   new CSS, as older browsers DO NOT use (read) the @media and will keep the original width spec, and all newer responsive browsers will use-read the @media screen where you can use the newer CSS references like width:100vw;   and   min-width:280px;


maybe like -
.blusec{width:20em;height:10em;}
@media screen and (max-width: 4010px){.blusec{width:100vw;height:100vh;} }

you will need to try different settings for the various layouts and responsive (and NON responsive for old) factors that are in your page, there is not a one size fits all solution for the vw  use that I know of, as having a phone responsive display will be dependent on the sectional relationships for width and height.

you can also use the way CSS reads the settings, you can place the older setting first, and the newer setting after, so the old will read and use the first and the newer will over ride the first with the last one it can understand -

<div style="width:100%; width:100vw;">

There is a polyfil for them
https://github.com/saabi/vminpoly

But you shouldn't be using them unless you can say you do not support IE8 at all.

@Slick812:
Javascript

    var browser=get_browser();
    var browserver=get_browser_version();
   alert('You are using ' + browser + ' ' + browserver);

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When ran from a vb.net custom browser, returns IE 7.0.
However,  http://caniuse.com/#feat=css-mediaqueries, shows IE 7.0 doesn't support @media queries.  It also doesn't support vw, vh units per this:
http://caniuse.com/#feat=viewport-units

However, the following code (first section in css file, last part shows HTML:

#leftcol {
	float:left;
	padding:5px;
	width:136px;
	height:100%;
	text-align:center;
}
@media screen{
    #leftcol {
		float:left;
		padding:5px;
		width:17vw;
		height:100%;
		text-align:center;
                background-color:red;
	}
}

<!--HTML code here-->
<div id="leftcol">
    some stuff is actually here
</div>

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Shows the red background even though it reports IE 7 which should not support and according to what I understand from you, ignore the mediaquery section.

I like your idea, but it doesn't seem to be working. Am I doing something wrong?

I can't seem to get the suggestion working, but a combination of javascript, php and dynamic loading of various scripts, with a page reload passing arguments to let the page dumb down to older browsers seems to work. It is not as good as the above would be if it had worked.

OK, I am on holiday now, and not in one location, little time.
 first I will say that I forgot to completely give all of the info about @media screen, The @media screen is supported WAY BACK, as it was for the printer page difference, BUT you did NOT look at the spec I gave -
    @media screen and (max-width: 4010px){
the  max-width:  is what actually what does it. This works on IE-9 and above, on iOS Safari 6.0 and above (iphone), and Android Browser 4.0 and above. Please Notice That I use a very high number here 4010px, as laptops and phones now have displays 3200 pixels , using 4010px will have this happen in all broswers that can read the max-width spec.
As a note - the vw and vh seem to be poorly supported and not only that,  seem to have a limited usefulness, I only fould a legitimate reason for using it as a FONT size adjustment, it was not a better solution for a <div> width or height spec, at least for me and how I do responsive pages.

Here is some additional Info, you CAN use subsequent CSS declarations to get backwards compatibility (I have already mentioned) this is an Actual css that I use in almost ALL pages I build -
.inblk {
  display:-moz-inline-box;
  -moz-box-orient:vertical;
  display:inline-block;
  vertical-align:top;
  zoom:1;
  *display:inline;
  }
using this I can Get the newer   display:inline-block;   spec in most OLDER browsers Including Internet Explorer 5.2. . .
Because, all browsers will start at the first css spec that they understand for  display:  ,  and then if there is another  display:   spec after it,  that it can understand (use), it will automatically use the later spec, not the one before it. This may be the BEST rout for you, , if you insist on using the poorly supported vh and vw CSS spec something like this -
     <div id="dvTop"  style="margin:5px auto; width:31.21345%; width:29.531vw;">
since the   width:29.531vw;  is after the    width:31.21345%;   , any browser that can use vh , will use that instead of the first percentage  width:   spec., and those that CAN NOT use  vh , will be using the percentage  width.

Also, if you are building web pages, you really should study and learn How, and Why to effectively use the media - max-width -
      @media screen and (max-width: 800px){  }
for me and many others, this opens up your ability to do responsive pages for laptops, tablets and very small cells

here is a site (and there are others) that can give you CSS , HTML5, Javascript, and SVG compatibility info -
    http://caniuse.com/

Unfortunately there are many, many CSS factors that can be addressed for various things to Fall back for old browsers. For me I avoid using the viewport  vh and vw, because of the few alternate android browsers that fully support it, and laptop browsers that do not fully support it.