You can do pretty much everything you need out of the box with Magento.
1. vendor page ("category" page) with list of products of that vendor
Make a “vendor” attribute for each product and turn on layered navigation. This will give you a “shop by” listing that can narrow down products by any attribute.
2. vendor/category page - list of product of one category of one vendor
Regular category management plus the layered navigation
3. ability to define special parameters for the vendors, .e.g.
b) URL of the vendor homepage
c) Long title
d) Short title
e) seo-title (used in addresses, see bellow)
f) return conditions, etc.
You can add as many attributes to an attribute set as you want. There wouldn’t be a centralized “vendor management” screen. But, by using the mass updater grid, you can search for products of a particular vendor and select them all and change all the attributes at once.
4. ability to use the vendor seo-title in addresses (e.g. product adress could be example.com/vendor/category/product-name ...)
You can define any SEO keywords for any product anyway you want. It would not have slashes, it would be all dashes like:
It won’t have a fancy module, but that shouldn’t be a roadblock to setting up your store. Fancy modules to help manage this stuff are a lot easier for a developer to write if there’s already a demonstrated behavior. In fact, I wouldn’t even know what type of module to write, since all the features you want are included by default. Sounds like you’re interested in highly specific views of this data (which you didn’t really post about). If you need anything else, keep posting…