Are they Mother of Pearl by chance? If so don't worry about them, or bone, or some other natural material?
I guess if depends on how you feel about gambling. Not knowing what you mean by "vintage" , I can't tell you for sure, or even maybe, and I can't be responsible for telling you to defy this law, but...... If you are talking about really old metal buttons, don't use them. If you think they are plastic from the 70's 80's etc, well I would take the chance, but that is me.
The gov has so many other fish to catch with real hazards that I am not sleepless over worrying about a couple of buttons by LaMOde on a dress. but that is just me. Am I telling you to ignore the law? No way!