I am a religious user of the Vasa SwimErg and coach with it too. I use the Vasa 2 times a week and swim in the pool 2 times a week, all of which between 4-5k each time and that seems to be the golden combo for me. I have noticed a big boost in my fitness and my swim form by using the SwimErg not to mention my back is ripped now 
I have been experimenting with the power curve and how to get an accurate critical power to structure workouts around. Be consulted with a few coaches and while it needs some tweaking the two ways that I like are the 5min 1min test which is similar to the 400/200 CSS test but you take the average then multiply by .80. Or the 1000m TT then use that as an average. I use damper door 3 for both tests. My only issue that I have with using it and TrainerRoad is the problem of two FTPs/CPs. It would be great to be able to set the Vasa workouts as swims instead of bikes too.
I create all my own workouts but would be happy to share. I like to focus on workouts that I optimize me being on the Vasa without flipturns and the added resistance so longer sets, force and power work, and aqua-velo bricks