Two screws instead of four causing excessive vibration? What about the one's that use ANY screws whatsoever? Then, they should be vibrating like hell.
I agree with onebuck, everyone is making a lot out of nothing.
My Jurassic-era Dell machine (over 12yrs old now, will be a teenager soon) had the original 80GB HD just slid in between two green plastic brackets with clips at the rear to hold the drive from sliding out. Oh yeah, and it's still working. It's been through hell (XP2000 :-)), Linux and now it's back to hell and still working.
Two screws, don't worry, it'll still work. It's the quality of the drive that will cause it to fail prematurely rather that the two or four screws.