right now I have one cable from the sub out to the mono input on the back of my HSU VTF2 sub. On the S&V video it shows that if the sub has two stereo inputs like mine I should use a y connector. Is my sub hooked up correctly? It sounds good right now, should it be even better with the y adapter?

No you do not need the y connector it will make no difference to your sound. I am surprized that the video shows this because in fact that is not the correct way to hook up the sub. If you are using the Sub out from your receiver then all the required signaling is in that one line out.

I think I remember the Hsu documentation saying that using a Y adapter will increase the level a little bit, perhaps making the auto on/off switch a little more sensitive.

I don't think that this is always the case. The documentation with my PSB sub claims a 6db boost with the Y connector.

You are getting a boost because you are effectivly doubling your signal input. To get back to the calibrated value you will have to lower the attenution on the sub. Using a Y connector will also make the sub much more sensitive, as the signal level is higher then it would normally be.