if the ticket price is less than 800 you cannot use it online i think because online it will cancel the remaining part. At least that's what i was told so i never tried. when i had a ticket that costs less the voucher's value i called to book and they created a new voucher with the remaining value. What is interesting is that the new voucher had again one new year of validity.
For my case i had voucher for a non working IFE in Feb 2016, i used half of it in September 2016, the other half has now a validity until September 2017.