I totaly agree with you and I'm glad you recognize SEO as not being a exact science (too bad there are so many that shout otherwise).
The more you know about it the less random things seem, like with everything (VM?). However regarding SEO there are a few things that I would say making at an unexact science. Maybe the most importatnt one being that it is totally subjective from the SE side. All rules are not open to take part of and so on. never mind...
But when you say this is not an VM question I disagree with you. For one this is the SEO, SEF section of VM so what better place to ask this question?
I didn't mean that is was in the wrong section or that we shouldn't discuss it. What I meant is that this is a general SEO question not specifically regarding VM. Therefore I think you will find better info in other places. I was just explaining because you found it so strange that no one replied, this is probably the reason I'd say.
More important there must be more people that have to decide if they will put all their info in VM or have a simple VMshop section and have all the info in Joomla as articles.
That might be another question to discuss but that questions wasn't here when I replied. It is somewhat off topic in this thread. Hmm, I think I read something else here, sorry.
I'm building a website with buying facility for an outdoor brand and I'm in doubt about having all productinfo in VM meaning lots of double content or having everything in Joomla meaning possible loads of backend work but less double content.
Why would you have double content in VM? You don't have to if you take care about where and how you place your products. I have chosen to go another way and rather be user friendly than SE friendly. That's just my choice. If you want both (which you are obviously gonna reply) you just have to take that old noodle for a spinn.

Read about double content and read about what ou can and can't in VM, there are plenty of options.