how do I add keyframes?

Download the Vegas Pro PDF manual and read it there. Adding keyframes is a feature that it's part of the standard manual reading and not something that others should be explaining before you've read it.

I have downloaded the Vegas Manual but it doesnt say anything about Keyframes in there.

I think I do know what keyframes are though. Is it when you add Video Events FX? However I can't find a plugin there which is a Zoom in or Ease in plugin...

I guess we have to ask you what you mean by "Easing In". To all the rest of us here, this means a slowly starting gradual transition from no effect to full effect, and can apply to any effect. You won't find an effect or plugin called "Ease In" because this isn't an effect; it's a method of using other effects.

Zoom is usually accomplished by using Pan/Crop or Track motion, both of which can be keyframed.

Easing in is accomplished by adding any effect you want to a clip on the timeline, then using a keyframe at the beginning to start with all the effect's parameters set at zero, and then another keyframe farther in with the effects parameters set to their final value. Keyframes are found at the bottom of each effect's dialog window in their on miniature timeline. Add them with the <+> button and move them where you want them. You can then click on each keyframe indicator on the timeline and set all the sliders and options for the effect for that keyframe independantly of the other keyframes. Vegas will gradually move from each keyframe's settings to the next when playing/rendering the result.