First of all. Reset the BIOS using the CLRCMOS switch or something similar on your motherboard. Then, put your 4 sticks of RAM back in the MOBO and go into BIOS. Vengeance supports Intel XMP profiles. Go to your RAM settings in your bios, and look for something called Intel XMP Profile. XMP is a profile for Ram timings and speeds preset. When you find it, click it and switch it to 1600mhz XMP profile. This is a very simple process usually requiring a few clicks. After you set the XMP profile, you should be able to see the RAM going at 800mhz in CPUZ
Vengeance RAM are underclocked by default, so you need to manually set it in the BIOS using Intel XMP profile. Hope this helps, and if you find a best answer, don't forget to select as best answer!