Probably a silly question, but can I have isolated .html pages as part of my website that are outside of the VSD project website? For instance, could I have mydomain.calendar.html that VSD will point to with a hyperlink and resides in the same public_html folder?

Sure you can. But why not place the calendar code on a page created in VSD and that way get the same design?

I would recommend Inger's suggestion but if want to do it with an iframe.

Use the code on this page
http://progrower.coffeecup.com/iframe.html
and add the page with the calendar to the root of your site with the add files tool. Now adjust the code example on my page to the name of your page.