Where ever you get it replaced 18 twists in the string, 11 twists in the cables and 1 twist in top yoke and 4 in the bottom yoke are the specs for the Invader HP. Make sure the cable slide is installed correctly after replacement and cams are in time.
Academy has changed several different bow strings for me. They may not have the string you need so you may need to purchase and bring give them a call
Bow or crossbow?
If you have a good bow press rated for Crossbows it really isn't different between a vertical compound and a compound crossbow. Main thing is the correct strings and cables along with correct length and timing.