Your client has a requirement that any employee will not get enrolled into the vacation...

Create a plan having a Waiting Period with UOM as Months and duration as six.

Create a rule in Accrual Matrix where you will check the length of service of an employee and If it is less than six months then accrual value should be 0.

Write Partial Period Fast formula that enrolls employees only after six months from their hire date.

Create a plan having vesting Period with UOM as Months and duration as six.