To keep the peace maybe you should try calling her by the name of Victoria there are many adults that choose to change there name for one reason or another. She may think it is more lady like than Vicky or she may just want to be different. If after you try calling her that you still do not like it maybe think about asking her if you could continue to call her Vicky around family and Victoria around co-workers and friends. There has to be a compromise in order for peace in the family to be kept. Just try Victoria for a while it may be easier than you thought to get use to using it.
I'm sorry I thought you had asked me if you should call her by Victoria or continue to call her Vicky. Explain to her that you just won't feel right calling her Victoria after a calling her Vicky all of these years. Tell her that you think it's okay to have the people she works with call her Victoria and that you find it silly to have family start calling her that also, tell her Victoria is so informal and that the family should be allowed to call her Vicky still, it's kind ok like a term of endearment between family members, she may then understand and allow you to call her Vicky still, you want to say it in a way that isn't confrontational.
Very nice choice of words and maybe you should ask her that the same way you have said it in your reply. Tell her that it is going to be really hard for people especially family to start calling her Victoria after calling her Vicky and that it isn't fair to those who have dearly referred to her as Vicky. Especially the older people in the family will not understand why the sudden change in the way you same her name and they will not want to change because older people are set in their ways and don't do well with change.