My 30 months little boy has a caught only at night and is breathing with his mouth open how can we get rid of his caught and is puting vix everyday ok?

Accompanying stuffy. /runny nose, fever - rectal temp 100.4 or above, wheezes, labored / rapid breathing, poor food/fluid intake = a set of symptoms that need evaluation. A night cough for > 2 wks. may signal mild asthma, for which he'd need bronchodilators. Don't give OTC cough/cold medicines till age 6. Vicks is useless & some kids eat it. Use saline nose drops, a vaporizer.& lots of Pedialyte/ clear liquids. .
Tonsils and adenoids. Child coughs frequently while asleep, may have sleep apnea. Sleep apnea is sometimes caused by enlarged tonsils and adenoids blocking the upper airway passages during the night, making it difficult to breathe. Mouth breathing is common with tonsils an adenoids. Allergies and post nasal drip another cause. You should check with pediatrician.