I am in a union shop, we get a week off every year for the annual shutdown. They do take volunteers to clean shop but if we sign up and dont get chosen to work we are told we are unable to collect unemployment benefits for lack of work because the union v

I work for a union shop, and each year we have a 1 week shutdown and per contract if we sign up to work the shutdown and are able we can save a weeks vacation. If we dont work we have to use that one weeks vacation if we expect to be paid. My question is if we sign up to work and nothing is offered it is considered lack of work.. Can we still collect unemployment benefits? I am being told no because we have to use our vacation for it if we dont work but even if we sign up and not offered? Is that legal? Im told because of the contact between the union and the company we cant.