here you go, I found a version of theX800XT bios for the X800 Pro VIVO Bios, thats been modded to use 16 pipelines, but only uses the X800 Pro speeds.
so if your Vivo is fine with 16 pipelines, but not the XT speeds, this bios should be perfect for you
I found it here, reading the details of this particular flashing method.
this quote caught my attention.
"If you know your card can't handle the XT PE speeds you can download a modified X800 Pro VIVO Bios from there as well (ATI X800 Pro VIVO 16P). This Bios is just 16P enabled and is recognized as X800 XT PE - clocks are unchanged"
I attached the bios here as a zip file.
goodluck, and please-post some results-I'd like to see