I remember when RX 480s came out, and they were getting ~19Mh/s on an 8gb card, then people understood the card's specs and now they hash at 29.
I don't think there is any single miner that is optimized for Vega yet, so it's using generic kernel. And it all depends on the algorithm and how it uses the card's resources.
The RX 480 since day one always did 25 mh/s because the first model to launch was with 8gb samsung memory and that is by default 25 mh/s, after bios mod up to 32 mh/s, bios mod came after a month or so.
The drivers are never optimized for mining at launch, neither are miners for new cards. We won't know the real hashrate for a few months until after the Vega gaming versions come out.