What are good cardio vascular exercises for an above left knee amputee?

I would suggest swimming as it would probably be the easiest for you. You can get an all over workout including your half leg. But other suggestions are a rowing machine (or actual rowing!) and a stationary exercise bike. These can be done using one leg and the rowing will help your arms which as an amputee you need. You can also get exercise bikes that the handles move on - this will help your arms. But any exercise that gets your heart rate going will be good. Try to find some time for weights as well.