That really only works with XP. VX handles autotiles a little differently.
However, if you Shift+click on a specific part of the tile, you'll be able to lay down that particular tile without the autotile establishing borders... So just Shift+click on the open grass tiles and lay them around the top of the building.
But, er, what exactly are you referring to? The grass, or the stepped appearance of the building? Need a little more detail.