If your delay is under 4 hours, the airline can lower the compensation by 50%. I assume your post contains a typo since the compensation for this flight is 600 EUR, not 800.

The airline can offer vouchers in a higher value than cash payments; this is what I suspect happens here:

> you have a delay of 3+ hours, which by precedent is equivalent to cancellation
> however the delay is less than 4 hours, which triggers the cause allowing the airline to lower compensation by 50% (for this type of flight)
> you are therefore entitled to 300 EUR pp
> airline offers you 500 EUR pp in vouchers as an alternative