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in the same C block i will have the main site, with no links to the vanity domains. these might get picked up by googlebot or they might not - it doesnt matter either way because my main purpose is to have humans directly entering the domain names be funneled to the main site.
does anybody foresee a penalty for using the above strategy to make use of my vanity domains? if yes, would constantly having (unique) new content on these vanity domains help avoid the penalty?
i just don't want these SERPS to be treated as doorways...
i dont even care if google doesnt index them
Then block googlebot with robots.txt -- and instead of index,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.