the two wires from the 47-2 were just part of the switch that cut power to boiler when water was low. I suggest just splicing those two together.
Next, if the PS800 cutoff is already wired, run a line and neutral from PS800 to vxt line and neutral. Then you need a wire from the W of the PS800 to the vxt feed.
Of course I recommend a good boiler guy do this or electrician, but if you are gonna do it yourself, http://www.hydrolevel.com/pages/pdf_files/vxt_120.pdf has the perfect wiring diagram for the old PS800 and new (page 2)