Can they take that time away with no notice? Yes, it appears to be an honest mistake. However, since you have planned the vacation already, will they let you take it without pay? That would seem to be a good compromise for the immediate situation.
Is there any time limit for them to ask us to repay them for their mistake? I assume they are asking you to repay the vacation pay you received in error. No, there is no time limit. You and the company should sit down and come to an agreement. If you were underpaid, you would expect the difference to be paid, right? Same for the company.
Am I entitled to a record of accrued vac. hrs? No, there is no legal requirement to provide you with your vacation balance, although a lot of companies that maintain the vacation recordkeeping through the payroll system do print the available balance on the check stub.
Most people know how much they are accruing each month, because they know how much vacation they get each year. Employees also know how much vacation they have taken, so it is not difficult to calculate how much vacation time is available. If you weren't sure how much vacation you had coming, it would have been a good idea to check first before you planned vacation time. 30 hours is nearly 4 days worth, which is a pretty big chunk of time no matter how much vacation you get each year.