Hi,
I'm sorry for the delay.
You cannot set the priority for ItemIDs in JoomSEF, but you can set which ItemIDs should be ignored. I'm not sure if it will work exactly as you want it, but you can try setting the ItemIDs that you don't want to be used in JoomSEF's VirtueMart extension parameters on the Common tab using the "Ignore Itemids" field. You can set multiple ItemIDs separated by comma.
From SEO point of view duplicate content (two pages looking very similar with different URLs) is not good. But if you pick one of the pages as default and create canonical link from other pages to that one, Google should accept it. But you would need to set the canonical links manually in JoomSEF.