Calling someone dumb is unprofessional only if the person is not. Otherwise is just impolite. And if we wasted time negotiating with the VC fund he has some opportunity costs.
http://www.acceleratingfuture.com/michael/blog/images/disagr...
Maybe the 'dumb vc' thinks you have what it takes to be successful as a person with the right product and that's why he/she wants to talk to you without knowing the product. If he/she reads this article you probably lost a fan.
Maybe he/she honestly just wanted to give good advice, if you think the advice sucks then you are the stupid one to meet 6 times with the VC..
If I say, "<Venture Capitalist X> had sex with a woman not his wife", then I'm being unprofessional because I'm taking someone's private matter and exposing it to the public, quite possibly maliciously. On the other hand, that's not what we're talking about. If I say, "<Venture Capitalist X> screwed me and damaged my career", I'm helping everyone else out there make better decisions. How is that a bad thing? How is that "unprofessional"? Relevant information should be shared. (It's the embarrassing but less relevant "dirty laundry" that shouldn't be exposed.)
If companies can damage our reputations by, for example, firing us after 3 months because we refused to commit perjury to save an imperiled executive's ass, then I think we should be allowed to fight back. They don't act like they're entitled to protect our reputations, so why do we have to protect theirs?
In OP's case, he's not even naming anyone; just pointing out sub-optimal behaviors.