I would vote for Nature Valley Granola Bar - Oats and Honey. I actually love those things.
Also, swap out some of the normal type chips for baked chips and/or pretzels.
As far as candy goes, 3 Musketeers are supposed to be better as less calories/fat with the fluffy stuff in them. Also, dark chocolate is better for you so maybe a bar of that? Not sure what may get in vending machine fare - maybe Hershey’s Special Dark or something along that line.
+1 Lara Bars. Not sure what kind of pricing you get, but they would be a good alternative to candy bars.
What about pretzels or a version popcorn? Not super healthy, but can limit damage done by chocolate bars and doritos. If they are looking to get a snack–they may be open to one of these intermediate options.
A Lara Bar has ~200 calories, with 100 of them from fat, plus 16-20 grams of sugar, 4 grams of protein, and 3 grams of fiber.
A Snickers Bar has ~280 calories, with 130 of them from fat, plus 30 grams of sugar, 4 grams of protein, and 1 gram of fiber.
The Lara Bar looks an awful lot like a candy bar, regardless of how they market and price it.
Popcorn?
We used to have microwave lite popcorn. Not the best snack, but the best that people will actually eat. The only problem is the bags are too small. Nuke the popcorn and try to make it back to your desk alive! NOT. Invariably someone says can I have some, until you get back to your desk with an empty bag.
Other ideas are granola bars, but check fat and sugar content. Get the healthiest the boys will actually eat. Some granola bar that’s as dry as Ghandi’s flip flop, will just sit in the machine until the cockroaches take over the world.
Mixed nuts with or without fruit also work?
We have “One Square Meals” here in NZ. They used to sponsor Hamish Carter. They are meal replacement bars in a 2 pack, that I see in vending machines from time to time.
I think the key is to not try to get the healthiest thing you can find, but he healthiest that the crew will actually eat. Rice cakes aren’t likely to cut it as a guess 
Fiber One bars.
If for no other reason than the name, the Aero Bar (chocolate or mint) should get a moment of consideration. Seriously though, dried nuts (e.g., Almonds, Peanuts, Cashews) were well-received in ours.
Just wanted to say thanks for all the great suggestions. Unfortunately the company servicing our vending machines is the biggest douche bag company ever. They are now not willing to make any changes. I just wanted to make a few small tweaks but they are not willing to do even the smallest things such as replacing one of the five different varieties of chips for a baked version of the same thing. I am so fucking pissed off. After all this, the ass hat company rep asked me if it was ok for them to install one more soda machine. Request denied!
If it was up to me I would fire these clowns but I guess they have a two year contract plus they service a second plant so it is not that easy. What I have done is to bring in fruit. Apples, bananas, pears and oranges. It’s the honor system. I have a jar next to the fruit. Each piece is $0.25. I also post the receipt so there is no question about anyone trying to profit.
Well, at least I tried…