Who sells lawn tractors with the Briggs Vanguard engine?

All the tractors at Lowe's, Home Depot, WalMart, etc that have Briggs
engines say "I/C" on them, not "Vanguard".

Or, are the engines that say "I/C" actually "Vanguard" engines?

You probably will not find the Vanguard at a big box store. It is
Briggs premium engine, competing with the Kohler Command. The Intecs
are good, usually outlasting the rest of the cheaper mowers. Some top
line machines use the Vanguard. Simplicity for example, but they are
usually only sold by dealers.
http://www.simplicitymfg.com/yard_garden_tractors.php