What's different about Vaughan is that there's a good reason (significantly lower taxes) for businesses to build big offices there. Even with North York Centre and Yonge-Eglinton, supposedly Toronto's "second downtowns", there's been essentially no employment growth since amalgamation because it doesn't make sense for anyone to set up shop there when downtown is barely more expensive.
The land itself is subject to supply and demand. There's no "subway surcharge". If nobody wants to build on a tract of land, people will buy there. What's happened on Sheppard is that, because of its proximity to both the subway and the 401, the land has much more value for residential development than for office development. There's a ton of demand from people who want to live somewhere transit-accessible but have a job that isn't.