Or…
Select from the Media Generators the Solid White.
Drag it on the timeline and place it next to the event you want the white fade applied.
Enable Automatic Crossfades from the toolbar (Ctrl+Shift+X).
Drag the solid white event to the left overlapping your video event and creating a crossfade for the duration you want.
that's what's cool about Vegas (and this Forum) you can do lot's of stuff so many ways - in fact most of it I discovered myself by using common sence - well with SOME help from this forum , uuuhhrrrummfff :-)
You can also change a dissolve to fade thru white. There's quite a few variants to the basic dissolve in Vegas that are worth a look at. A Chroma dissolve is well, rather interesting!
Bob.