If the vanity domains contain keywords that are potential type-ins, then a 301 redirect to your principal domain will get you some mileage from those domains. No need to host any content at all.
Then block googlebot with robots.txt -- and instead ofindex,nofollow use noindex,nofollow. The fact that you chose index,nofollow is the indicator that you are hoping to get some search engine action from those domains. Also you mentioned "optimizing" - so of course you are looking for some traffic from on the SERPs.
But as you are already aware, your idea is to use these domains as holders for doorway pages. Today's search engines will not like that - it's just not 1997 anymore. It's not a viable idea.