If you have to keep topping it up then there must be a leak somewhere either in the boiler or pipework. I wouldn't run it at zero pressure, keep it topped up to 1.5 or max 2.0 bar.
I would request a call out from vokera to look at it, and IF a part in the boiler is the cause of it, then they should at least replace the part free of charge even if 2yrs out of warranty as part are expected to last longer than this.
I had a similar problem with same boiler some part had corroded prematurely and it was replaced free.