At this time, Visual Site Designer is a purely WYSIWYG editor, which means it cannot be used to directly edit HTML. Additionally, it can only work with the VNU files created within the program. You can insert HTML into your webpage using the Insert HTML tool, but designing an entire page that way would be extremely difficult.
If you experience a computer crash or accidentally delete your VNU file, you may be able to recover your website. Read What Happens if I Lose my .vnu File for more information.