I remain in the phase where I pretty much only power up to L30. Pre gym rework it made great sense to max out your best mons. In the raid meta it makes much better sense to have a LOT of quality attackers so you can assemble high quality raid teams for anything (sometimes with a high quality second selection as well).
Of course, once you've reached that point, then feel free to max out. I probably wouldn't look to Vaporeon first though. A nerf in the future wouldn't shock me one bit.