Look on page 21 of the Onkyo manual. It shows how to connect the sub.
Velo is high on using a Y splitter, but it isn't needed. It won't hurt to use one and can help with the auto on function.
From the VRP1200 manual:
Your new subwoofer is equipped with speaker-level and line-level inputs. Use the LINE LEVEL jacks when connecting your subwoofer to a pre-amp, signal processor (such as LFE out), linelevel crossover, or receiver with pre-amp level outputs. When using the line level jacks, some receivers may not provide enough signal to have the unit’s auto-on feature operate properly.
Additionally, this lack of signal may also cause the subwoofer to produce less output than it is capable of.
To alleviate this condition, we recommend the following steps:
1) If using line level jacks, BOTH THE LEFT AND RIGHT INPUT SHOULD ALWAYS BE USED
– never use just the left or just the right input. If the subwoofer’s input comes from the
LFE channel of your receiver, use the included “Y” splitter to divide the single input into
two inputs for the Left and Right subwoofer inputs. If using line level connections from
a preamp or signal processor, use both Left and Right inputs from your preprocessor or
preamp and connect them to the Left and Right inputs on your subwoofer.
And you should be using the Line Level inputs on the Velo, not the Speaker Level inputs.