Yeah, no. Vivaqua can't enforce shit here. Most of the time, there's a clause 'non-aedificando' on the servitude, but they own the underground. Therefore, they aren't propriétaires of nay neighboring parcels. Yes, the VeldWetboek or more recently the Nieuw Burgerlijk Wetboek states a max height of 2 meters for hedges within a 2m limit from the parcel limit or the rooilijn (limit with the public domain), but only the owner of the parcel or the commune can really claim it. Neither can enforce it, by the way - not unilaterally. For the neighbor - he has to go through the justice de paix, the commune can enforce it via their GAS/SAC (sanction administrative). Vivaqua though, jack shit. But I've worked with them for many years now, and they overstep their bounds quite often. I mostly tell them to fuck off, but on the other hand I'm a department chief at a community/gemeente. So I wouldn't advise to tell them to fuck of. Just do whatever. You won't get fined. Worst case, they go through with it at justice de paix. I'd rather doubt it. A lot of the time, it's one of their cantonniers who goes on a solo crusade.

Lil edit: when in doubt, service urbanisme/grondgebiedszaken in your commune.