Hi Mark,
Looking at the htaccess file, to get it to work as-is without the "vitamin/" part, it'll end up rewriting *everything* as though it's a product. It looks like the "vitamin/" part is a bit of a dirty hack to make sure it only treats the vitamin category as a part of the "shop".
What you could do is replicate this functionality but replace "vitamin/" with "Joints/".
Downside being you'll need to do this for all ailments.
eg:
RewriteRule ^Joints/?$ category.php?category=Joints&page=1 [L]
RewriteRule ^Joints/index.html$ category.php?category=Joints&page=1 [L]
RewriteRule ^Joints/([0-9]+)/?$ category.php?category=Joints&page=$2 [L]
RewriteRule ^Joints/index([0-9]+).html$ category.php?category=Joints&page=$2 [L]
RewriteRule ^Joints/([A-Za-z0-9_-]+).html$ product.php?category=Joints&product=$2 [L]
This will prevent it from trying to rewrite other urls as products, protecting the integrity of the rest of your website.
Downside being that you'll need to replicate the example above for *every* category on your site... eg Joints, Muscles, Bones, etc.
Let us know how you get on with it. I'm no mod_rewrite guru, so my code might not work "straight out of the box", but it should give you an idea of what to do if not!
Cheers,
Karl
web design, development & seo by DigitalVibe