You should be able to tie into the waste line for your new stuff, but you can't use a waste line as a vent. You might not need a new vent all the way through the roof, but you need to connect to the existing one above the drains of those above you. The general rule is 42" above the flood rim (say the sink upstairs was at 36" for a kitchen counter, you couldn't tap into their vent until it was at least 42" or 6" above where it could flood). It might be easier to make the connection in the attic, or make a totally new run all the way to the roof; can't tell without being there.
If the plumbers properly pre-plumbed in a vent line, and work above it has not compromised it, then you can use that as your vent. Again, can't tell without being there. Can you take a picture and post it of the vent? Does the pipe labeled vent continue down, or does it just terminate? If it is just terminated, it probably could be used. When you cut the cap off, make sure you have a bucket as it could be full of stagnet water that accumulated from condensation and rainfall...it could be quite gross.