There won't be any real difference in sound between down-firing and side-firing, nor will it make a difference to your neighbors where ever they are with respect to you. They will hear the bass if the walls are not real thick. Bass can only be hindered by very thick and dense barriers. I love the VTF3, however you may not be able to use all that power- at least if you are intent on being a considerate neighbor. The Hsu subs may be a good choice because you can adjust them to minimize the deep bass and put more emphasis on the mid and upper bass, which means less bass frequencies which can leak into your neighbors' home.
One thing you should think about doing, if you are able, is using the subwoofer in a near-field placement. This is where you place the subwoofer in a location very close to the listening position, like right next to your seat as an endtable or perhaps right behind the seat. This way the sub doesn't have to play as loud for you to get a nice bass sound at your seat. This will be doing a favor to your neighbors and also a favor to yourself. Another advantage of this placement is since the sub doesn't have to work as hard, it will produce less distortion and give you cleaner bass. Also, as a corollary, it will last longer since it does not undergo as much mechanical exertion or generate as much heat. I use VTF3s as endtables with the drivers blasting right into the seat, and that gives a punchier sensation as well, as opposed to a more distant placement.
