Lens for Nikon D3100 - VR/OS vs Price Range?

First time posting a question here, hope to get some great responses: D

So I'm new to photography, I purchased a Nikon D3100 two months ago with an 18-55 VR kit lens. I'm now searching to add two more lenses to my arsenal to help me learn.

For the first one I have already decided the Nikon 50mm f1.8, but for the second I was looking for a zoom lens. Particular Sigma's 70-300 APO
http://www.adorama.com/...AMNKG.html

Now this does NOT have Vibration reduction/ Optical Stabilization. The version of the 70-300 that does is priced at $369-$399
http://www.adorama.com/...OSNKG.html

Now, as a beginner, that $400 mark is beyond me, I'm not going to dish out that much, I can't.
Is the VR Really worth that mark up? Wouldn't a tripod do the same thing? What situations require VR?

If VR is really important, would I be better off going with Sigma's 50-200 that has VR/OS http://www.adorama.com/...45NKG.html

And also, since I have only listed Sigma, a third party lens manufacturer, is there another brand that may be cheaper and better? Nikon's own lenses do get very pricey…

Thanks!

Added (1). In response to the_camera_guy, I will hardly use autofocus: D Really the only time I set up autofocus is when I give the camera to my mom to use xD

So the ability to autofocus or not is not a big deal for me. The only situation is may be useful is when I use my D3100 to shoot skits, sketches, and short films with my friends. I know of that internal noise which is why I have a Zoom H1 external recorder, heh.

Out of the three Sigma lenses you listed, only the 50-200mm is able to autofocus with your D3100 because it has a built-in HSM motor (other lenses all rely on the camera's motor to focus, which the D3100 lacks). As you mentioned, the 50-200 also has OS, which pretty much automatically makes it the best choice out of the three lenses.

Another lens you should really consider is Nikon's own 55-200mm VR, available here for $220: http://www.adorama.com/...00VRU.html