If it were my Vibe, here's what I would do...
1. Wash it with
Mother's California Gold Car Wash or
Mother's California Gold Caranuba Wash & Wax.
2. Dry it.
3. Use the
Mother's California Gold Clay Bar System. I use it on the entire vehicle, including the windows, headlights, fog lights and taillights. Most people simply wipe each area down with a clean towel after they use the clay bar, but I wash the vehicle a second time. It may take longer, but I feel it does a better job of removing all the junk the clay bar just removed from your paint, windows, lights, etc. This part is entirely up to you.
4. Apply and remove
Mother's Ultimate Wax System Step 1 - California Gold Pure Polish.
5. Apply and remove
Mother's Ultimate Wax System Step 2 - California Gold Micro-Polishing Glaze.
6. Apply and remove
Mother's Ultimate Wax System Step 3 - California Gold Pure Brazilian Caranuba Liquid Wax.
This is a rather long and tedious process, but it should get rid of the yellowing and dullness you described. Unfortunately white paint is prone to showing staining, etc. much more than any other color, so it takes a little more work to keep looking good.
I do this entire process on both our Vibes twice a year, once in the spring and again in the fall. I will actually be doing the fall treatment pretty soon. To maintain it, I use a coat of Step 3 every month or so.
Best of luck restoring your Vibe's appearance
.