If your baby has a diaper rash you should use some diaper rash cream to help clear up the diaper rash and also allow the baby or child in diapers to go diaper free for a few hours each day.
If the toddler is active then they may pee on the floor or poop on the floor and it has happened with my kids but I just clean it up.
If they are sitting watching TV and I had them go without a diaper I would sit a puppy training pad down for them to sit on so that if they peed or pooped it would get onto the training pad.
Letting the baby or toddler go diaper free for at least a few hours per day can help air out the skin and help the diaper rash go away sooner.
But yes you can also use Vaseline to help lock in moisture on the child or babies skin and to also help make a barrier against their skin so that the pee and poop doesn't come into contact with the babies or kids skin.
Make sure to change poop diapers as soon as you notice but give them 5 or 10 minutes to finish pooping.
As for wet diapers those need changed every 2 to 3 hours depending on how much the child or baby has had to drink and how often and how much they wet their diaper.
If they don't wet much then you don't need to change the diaper as often and you don't have to change a wet diaper right after one pee as they can hold up to 3 and sometimes 4 pees before the diapers need changed.