Adding an effect to just a portion of time
This is VERY simple once u've learnt it, the problem is I don't think the Sony Vegas help describes it any good.
First, start of with putting the marker where u want the event to end. (ofc u should start with adding an effect, but hopefully u realized that too)
Mark the clip.
And click S
Now u parted the clip in 2 bits and the effect u added will only be in one of the clips and not the other, thus only added on a portion of time. And if that didn't help, here is where u can read it in the Sony Vegas Help
Reversing a clip
So easy it was to reverse a clip!
End of part 2.