I honestly can't say which of ParkMGM or Vdara is a closer walk to the Aria conference center. I can say that I've stayed at both hotels and like them both very much. They are my first two options when I plan a Vegas trip.
Vdara is non-smoking and does not have a casino, so it's not quite as crazy in the hotel lobby. There isn't a full service restaurant on site, but they do have a 24 hour market/cafe as well as a lobby bar and lounge. The base rooms are all studio suites and spacious, nearly 600 sq ft. Even if you won't need the mini-stove, the mini-fridge is convenient if you want to keep some bottled water or other beverages handy in your room. And it's just a couple minutes walk from Vdara over to Aria or Bellagio.
ParkMGM is also non-smoking, which makes for a nice casino atmosphere. The casino floor isn't as big as Aria, but that makes it easier to navigate. ParkMGM also has a nice choice of restaurants. The rooms are smaller than Vdara, at a little over 400 sq ft. The rooms were renovated when they rebranded the property a few years ago, so they should be in good shape. My only minor pet peeves are the bed is a little lower to the ground than I'd like, and there is not a ton of closet/storage space in the room, especially compared to Vdara. Still, neither issue is a deal breaker for me. Finally, the walkway from ParkMGM to Aria is inside, so you can avoid the Vegas heat when walking over.