While the URL does play a part in SEO, don't underestimate the shopping cart software. It does very well in the search engines, especially if you use the SEO tools in the program. Just wouldn't want you thinking you won't rank at all.
To give an example: I had two sites running for some time, one built with another program similar to VSD - a static site, and one built (with a new URL to boot) with Shopping Cart. Within 3 or 4 months, my shopping cart site was outranking my older static site.
So you have to ask yourself if the learning curve, and extra work involved in setting up both a VSD & Cart site is worth it to you. I agree the URLs are a nice advantage, but it depends on how much time you want to put into it.
If you are considering VSD in conjunction, I would suggest looking at the mega pack for $289 on the Home Page. You basically get it, and a whole heap of other useful programs too, for $60 more than the PRO eCommerce pack.