Ok I managed to do this without a spline. I just use a start and end point (2 vectors) and then I just calculate the difference between their 2 relative location, multiply that by a ladder progress float which defines where the player is on the ladder (pressing up increases value, pressing down decreases value) clamped between 0 and 1. Then I just multiply ladder progress value with the difference between the two location and add that to the lowest location and use that as relative location for my player location. Works as it should. 
