Hi Rain,
What Eric says is not only probably best, it's the only way! VSD can't do responsive/fluid/liquid designs, only fixed ones. You'll could use the HTML editor for a, as you call it, flexible design. The HTML editor comes with some responsive templates, and you can find many of these searching the web.
The last website I did this way was
www.doehetsolar.nl . Like Eric suggested, it uses a script to determine on what device the site must be shown. The main site, a fixed one, was made with VSD and the other version for Android phones and tablets, iPhone, iPad, Windows phone, Nokia, whatever, with the HTML editor.
Success, John
BTW, some time ago (a year?) someone suggested Coffeecup could make a Visual Site Designer PRO version, that might do (among other things) responsive websites. I was willing to pay $ 100,- for that option alone. This suggestion has disappeared in the digital mist, I suppose...
John van Hulst