If I understand you right, you have regular 1/8th " headphones plugged in to your PC but you want to use your speakers for vent?

Both vent and windows can make this work. I would setup vent. Just go under setup and change your Output device. It will either be named "Something or other Speakers" or your sound card. If it's your sound card it can be a bit tricky. If one don't work, try the other.