VirtualBox is not in the same playing field as VMware but it does work well and is popular etc. The team already knows how to build virtualization software. To me they're just missing some management tools.
Big companies like Google and Oracle don't really think about what's coming next year. Sometimes they invest in something that will pay off big in 10 years. I think this is one of those deals.
This is pure guesswork obviously. I have no idea...
First, I think many engineers at Oracle use it to test software locally without having to fallback on complex solutions. Second, it's probably a small development team, maybe three or four FTE, and how much money is that for a company like Oracle? VMware licenses are probably more expensive.