Basically merge mining works like this:
When a miner mines a block for LTC, it will get LTC rewards. If he is merge mining for VIA, that solved block hash is accepted by VIA blockchain as ProofOfWork (not 1:1, depending on the hash complexity, e.g. solving 1 LTC block grants you 3 VIA blocks - this is ultra simplified, just for giving a rough idea!).
So even if the reward from VIA goes near zero, it is actually still cool for a miner to do merge with VIA cuz he doesnt have to use any resources to get a small additional reward (plus the fees).