You can mix old piping with new piping on your plumbing system. We often use abs with cast iron. You cannot glue abs and pvc together, but you can connect them together with the proper transition couplings.
The vent for the bathroom does not need to go all the way to the roof, it can connect to a vent in the home at another location, but there are a few requirements.
1. The vent needs to be the same size or larger than the vent for the toilet.
2. The vent must tie in 6" above the counter top of the sink.
3. The vent cannot be run horizontally below the top of the sink.
4. The vent must allways run up towards the roof and when run horizontally must pitch slightly back towards the drain.
Typically a toilet vent by national code is 2" in diameter and a kitchen sink vent is 1-1/2 in diameter. This would normally not be an allowed connection. If the sink vent is 2" it is acceptable.