Create a new, custom "default widget" for the vtiger home page.
We wanted to do this pretty badly. However, the vTiger documentation on the code sucks dead bunnies and the only post anywhere about how to do it was by someone whose native language was not english and who assumed "rather a lot" about the environment you were putting the modification into.
This post documents our process to get custom widgets running in vTiger 5.4.0. I make no guarantees that I didn't leave something out, but I tried by golly, and it's pretty good documentation as it sits. Good luck and happy programming!