As long as the toddler is 2 years old or older you can safely use the Vicks Vaporub on your toddler.
If your baby or toddler is under 2 years old then you should never apply any Vicks Vaporub on them because it make them very sick and be potentially deadly to them.
When Vicks Vaporub is used in toddlers under 2 years old it can increase mucus production and inflame the airways and make it difficult for the toddler or baby to breathe which can be deadly.
You can however purchase and use some zarbee's chest rub that is safe for infants 2 months and older to help with a toddler babies cough or congestion.