Depending on how long the boot was bust...
...and what kind of crud was in there, it may not need rebuilding IMO.
I've worked on several VX's that have driven for a while with busted boots...
...and the grease was mostly gone, with what was left being ALL kinds of dirty.
A real good clean out..check the balls and races for uneven wear/dings etc...
..and ya might just be OK.
Jo