iPhone + Messages Beta: prevent iPhone from vibrating for every message?
I'm afraid that's not possible in the current iOS.  

But there are a few work-arounds:


You can define a specific vibrating tone for that specific contact person which will do almost nothing (unlike the default long and hard vibrating)
or you can disable vibrating when you enter Silent Mode and when you start having this rapid-fire conversation, just switch your iPhone to silent mode and no vibrating will happen!