You can definitely password protect and work online, but Vision is a self contained Ruby on Rails server giving you all the same capabilities as through your store.
I’m not to sure what else you’re going to do with Shopify other than “design”. You can’t even access the application code as it resides on Shopify’s servers and is not available to the public. This is a hosted application.
Well, what I’ve had to do is register for a second store, password protect it, and use it as my test bed. I tried using Vision, but as soon as I added a new link list to my store and downloaded the code for example it broke… Also I wanted to try different aspects of html, and images, and tables in my pages, and that became too difficult.. So I’ve found having a second store really solves all that.