Yes, whilst they can't force you to take a benefit instead of a contractual cash/salary payment Sometimes employers have chosen to give benefits in lieu of a salary INCREASE as part of a periodic (usually annual review)- eg a below inflation salary increase + gym membership subsidy +extra day's holiday. That sort of change is of course perfectly legal.
Holiday vouchers are not something I've heard being given before as a regular element of pay but they are sometimes used as one off rewards