hiya Javier,
Just remember that if you do open it in HTML Editor ( @Tom, he never said anything about opening it in the Visual part of the editor lol, not sure where you saw that ), that if you change any of the markup at all, even spelling errors, you cannot get that back into VSD. Means you'll have to then take the changes that the HTML Editor makes and change them in VSD to put the changes into the .vnu file or you'll have to make these changes every time you save your site in VSD. What you're suggesting would "only" work if you don't plan to edit it in VSD anymore and plan to carry on the editing in HTML Editor (or similar code editor). Hope that makes sense.
You would be better off to use a spell checker in a regular text editor or even if you do do it in HTML Editor to find the problems, and just change them in VSD at the same time.
Having said all that, doesn't VSD have a spell checker? If not, it's sorely in need of one I think