Merger and Vesting question

[Guest Lisa A Allen]

Employee works for company A for five years. Company A has a six year vesting schedule. Employee leaves company A to go work for company B. Company B has same plan and same schedule. After the employee has worked for company B for a year, company B purchases company A. Company B offers company A employees retention of prior years of service for the purposes of the 401(k) plan vesting. Is this employee out of luck and back at one year of service or is company B required to recognize employee's five previous years at company A? Or is it up to the company to recognize her service at company A or not?