You can check out the recipe I started with on this site: http://www.miiamonthly.com/2010/01/03/soda-pop-recipe-new-york-seltzer-raspberry-flavored-clone-v5-and-v6/
I went for the vanilla version which was always my favorite. It turned out more difficult as I don't have measuring cups and was trying to reverse engineer a flavor I haven't had in 20 years. But all in all it was a good attempt. My recipe turned into Canada Dry Seltzer, Simple Syrup and Torani Vanilla flavor. The mix was entirely by eye so I can't give you anything exact.
It's a pretty difficult combo to pull off because you are going for something pretty sweet and vanilla'y but also very crisp and light. Too much of the syrup gets murky flavored overpowering the vanilla, but the original flavor was quit sweet so you need it there. It may be a smaller amount of more concentrated syrup with vanilla extract is required. Also a soda siphon may be required to get the right fizz.
I really need to get my hands on an old bottle to check out the ingredients list. I am not entirely sure Seltzer was the only ingredient it may be a bit of phosphate in there too, which would make it closer to a club soda base. Color is another thing eluding me as its naturally colored. Maybe I should be cooking down sugar water and fruit compote to get a better favoring agent.