How do I silence a call with my Samsung Fascinate? I want to be able to silence the ringer/vibration of a call without hitting the "ignore" button.  

The reason is that if someone gets sent to voice mail after one or two rings they will likely know that you chose to ignore the call, however if you just let it ring until it gets to voice mail there's no way to tell if the person ignored you or truly missed the call.  I'm not in the business of ignoring calls but there are times I do so and I don't want the person to know that I didn't want to talk to them.  

I never use a ringtone, but when my phone is sitting on the desk the vibration is LOUD!  Every other phone I've ever had allowed me to silence the vibration by hitting the down volume button.  This doesn't work on my Fascinate.  Another oddity is that I get two different screens when people call: 1. a slider option to answer or ignore with an option at the bottom to "ignore with text" 2. or three buttons: Answer, Ignore or Silence (but this one is always grayed out).  There doesn't seem to be any order to which screen I'll be presented.  

Any ideas how I can start silencing calls?

I've got a rooted Samsung Fascinate running Froyo on the SuperClean 2.9 ROM with VZW.
You can silence the call without rejecting it by pressing and/or holding the physical volume down button.

Note that some phones accept a single momentary press, whilst others require you to hold the button for a moment or two.  It seems Samsungs tend to fall into the latter group.