Huge up and downs after vbseo 3.6 and sitemap 3.0

I installed sitemap 3.0 in november 2011 and got a nice UP in visitors in the next 2-3 months instantly.
(not knowing if it was that, but ...)

Then I installed vbseo 3.6 after 3.52 (on a vb3.7.2 PL2) in march and instantly fell down deeper than before november.

Did you noticed something like this on your site?

I'm looking at the panda too, but don't believe it's coming from there.
Any idea?
Did I miss something here .. I should know about?

Thanks a lot!

Marcel

nov 2011 hasn't happened yet

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