I would prefer assuring Vargas playing time and Mauer filling in around him vs the other way around. This is a perfect time to do it. At the start of next season we would have a clearer picture of him with consistent playing time under his belt.
IMO there are still plenty of at bats to go around even with Mauer being the everyday starter at 1st.
-Defensively Mauer is one of the best in the league, when you have Polanco at SS and Sano at 3rd, both of whom are still a little "iffy" it is imperative you have Mauer and his ability to dig out the tough throws. Vargas, while not Ortiz like at 1st base, is closer (at least with my eye test) to below average defensively at 1st.
-As mentioned above, Mauer had a horrendous start to the season, but since then has turned it around: .797 OPS in his last 25 games and a .897 OPS in May, most importantly in his last 25 games or so he is getting on base at a .363 clip.
-I'd prefer that Vargas gets 4-5 starts per week at this stage. 1 at the "cost" of Mauer, 1-2 at the "cost" of Grossman and 1-2 at the "Cost" of Rosario or Kepler (against LHP). Very doable.
If the Twins fall 10 games out of the hunt, then yes, he should probably be taking some more at bats away from Mauer, but while they are in contention? nah.