This is a very hard conversation to have with anyone, but it will be especially hard for someone on a ventilator. If your parent can communicate with you, I would hospice.
try to talk to them to see if they feel they have a good quality of life. Ask yes-no questions like, "Are you happy?" or "Do you ever wish that you were not on the ventilator?". Depending on their responses, you can direct your questions toward discussing
Another suggestion I would have would be to contact the hospice organization (ask your parent's physician/ nurse/ social worker for the information). Many of the hospice organizations have support services, and they can help you with your discussion. Perhaps they could even come with you to address any concerns that you may have in front of your parent so that they can be part of the decision making process.