What can I do if my employer sold the company and the new company took my earned vacation time because their vacation is different?

First, unless you had a written employment contract, employers can change vacation policy, including vacation carry-over or carry-forward policy at will--though "at will" is a slight misnomer here: if the vacation days were earned (e.g. earned at the rate of 1 day per month of work), then while the company can change its policy and eliminate or restrict carry-forward, it generally needs to provide some transitional time to allow employees to use the days. It's common, for example, to give employees a 3-6 month transition to use days in surplus of those they can carry under the new policy. A failure to allow any opportunity to use earned days could give rise to liability--e.g. the abilty to sue the employer for the value of the days. (Though, of course, since suing one's employer is a drastic step, it's often not worthwhile to do this.)
However, there's another issue: how was the company sold? IF the company was an LLC or corporation and the new owner(s) bought the actual LLC or corporation, you are still employed by the same "person"--the same legal entity--as before, and the discussion above would apply.
But if the company had not been an LLC or corporation (e.g. it had been a sole proprietorship or partnership), or it had been an LLC or corporation but the LLC or corporation itself was not purchased, but rather the buyer only bought the "assets" (e.g. name, intellectual property, customer list, accounts receivable, inventory, equipment, etc.), then you are working for a new employer or person--it may feel like, and for most practical purposes be as if, you are still working for the same company, but legally, you're working for someone new, the same as if you changed jobs. In that case, the new company or employer is not obligated to honor any committments of the old employer other than ones it chose voluntarily to take one, and so would not be required to honor you old vacation days.