You can absolutely relocate this pipe, but you can't use 90 degrees. The fitting you use is a 45 degree fitting. You can have no horizontal section of vent pipe below the levels of the fixtures. Vents have to run straight up to the roof at no less than a 45 degree angle from the vertical till they are at least 6 inches above the overflow of the fixtures. For a sink thats at least 42 inches above the floor. You can diviate them to the outside by using 45 degree fitting. Cutting the cast iron is a real trick though. You need either a cast iron soil pipe cutter (a chain snapper), a band saw with a new blade, or a carburendum chop saw. Cast Iron is very tough, I've seen people try to cut it with a saw-zall and take a long time (hours) and many new blades. Even the blades they claim will cut cast iron takes a long time. A soil snapper takes one minute.
Once you get it cut, you can use a Fernco or Mission stainless steel transition coupling to go from the cast iron to the plastic. Just remember cast iron and plastic have two different outside diameters, even though the inside diameters are the same. If you try to use the wrong coupling, it will not seal properly. Mission and Fernco make the proper couplings to transition from the different outside diameters.