The Pioneer site lists IR codes for various components.  The VSX-1015 isn't listed but 1016 is, and I used Remote Master to find that the Advanced Code for Signal Select is 39173 using setup code Audio 0300.

Setup code 0013 sends plain Pioneer protocol, device 165.  0300 sends Pioneer 2CMD protocol, which also uses device 165, but sends two command bytes instead of one.

You can assign 39173 to a button by temporarily assigning 0300 to Audio (press Audio,  setup until 2 blinks, enter 0300, should see 2 blinks) and then press setup until 2 blinks, enter 994, press setup briefly, enter 39173, press the button you want to put Signal Select on (2 blinks).  You can then re-assign 0013 to Audio, but the advanced code will still be assigned.