Use file manager and go to "/system"
Mount the folder R/W(Read/Write
open build.prop in Text Editor
look for "ro.horizontalVvvm = True" and chenge it to False
look for "ro.horizontalBUA = True" and change it to False
save your edit, and back out to thr "/system" folder. Once again switch the directory back to R/O, exit your apps and reboot.
You shoulkd now be able to install your Vvm.apk without it telling you to uninstall it because one is already installed.
Note I am using Virtuous rom however it should work for most roms I would imagine.
If you do not have a Vvm.apk here is one from the Droid X but it works great on my incredible. Looks almost like the stock incredible version.
URL=removed by me cuz im a noob and external links are banned for us as noobs
Hope this helped.
FYI i know this is an older post, but maybe, like me, someone will search for how to make vvm work - this is for my tbolt, but looks like it works on andriods in general, just gotta have hookup to big red.
Im running synergy rc2 and these switches are already true. i used root explorer to browse to the system/build.prop file and viewed it (didnt have to edit, but if i did, root explorer already opens r/w defalut and if you select and hold, the menu that comes up has the 'edit in text editor' selection available)
the titanium backup of vvm.apk wouldnt install, but when i downloaded the one above (THANKS FOR IT

), i installed it and my messages started coming in. i installed it right from the SD card - not sure if that will bite me in the butt later, but oh well.
for those who ask why vvm - i am new to android and never set up gvoice account before jumping into the root game. oops on my part, but its kinda nice cuz it makes me dig in and look at the way things work. to retreive my vms, both past (like my 4year old son calling and telling me that when we went through texas, thats where t-rexes lived long ago) and present/new. now that it is working again, NOW i can migrate to gvoice and drop the 2.99 fees!
i suspect there are others in a similar boat...