Remember, 'purity vows' are a man made concept. Not saying its a bad idea, but its really just a ritual and has no real power or biblical basis behind it. What's important is not taking some trendy vow, but the commitment you make to God to live your walk out in every aspect of your life, and living that promise out. God has forgiven your past, you know you want to do right in your future, so do it. What holds people to their convictions is not making vows, its determining you want whats best for you, and to honor God. People make marriage vows all the time and look how few are ever kept. If you want to stop having sex before marriage, then stop.
Don't worry about the whole 'i had sex before, can i still make a purity vow'. Everyone will have different answers. All because there is no right one because, as stated, this is man made, and things like this only complicate simple issues. My thought is, make your vow to God. That doesn't need a public ceremony or a pointless ring. Commit to Him for your sake, not as a show for others. If you think the ring, or some other item, may help you as a reminder, then go for it, but no need to publicize this.
To me these are kind of a spiritual pride. People getting up in front of their church, or whoever, and saying 'look how spiritual i am for doing this' kind of mentality. And everyone fawns all over the people for making this 'vow'. It seems silly to me.