I don't see VEGA having a noticeable effect on ETH mining - same reason the Fury is not much used for ETH, the HBM memory latancy kills the hashrate.
 I DO see a high probability of VEGA being a good mining card for some OTHER stuff though, like ZEC.



 For the small windows - build an air box around them, probably need to use 2x6 or so, with the box being a little larger than the size of your fans - no need to get fancy.
 Then cover the end of the box with plywood, and cut a hole in it to fit the fan.
 Your airflow is still going to be limited by the size of the windows though - and 28 GPUs is probably still going to run pretty hot even if you use a high-end gable fan (there are 16" gable fans rated for 3000+ CFM, and they're designed to handle SOME backpressure in most usage.

 The Rotron fan Phil linked should be a good, reliable unit - I've had Rotron fans go 20+ YEARS in 24/7 usage without issues, and I've got ONE of them that's pushing 35 years old now, and just STARTING to see some bearing wearout problems.[ b]I just don't think 455 CFM is going to be nearly enough to handle the number of GPUs you're talking about.[/b]