This is how the scam works:

If you stay one year, you won't get vaction pay.

If you renew the contract for the second year and return to Oman in September, you'll get vacation pay for the first year, but not for the second year.

If you renew the contract for the third year, and return to Oman in September, you'll get your vacation pay for the second year but not for the third year.

In other words:

If you work one year, no vacation pay.

If you work five years, you'll get vacation pay for four years but not for the fifth year.

If you work nine years, you'll get vacation pay for eight years, but not for the ninth year