I would change food if possible anyway, Purina isn't particularly high quality food and it is very hard on their system to digest especially the dry, which may be the cause of the vomiting.
Dog Food Reviews and Ratings | Dog Food Advisor
is a website that you can use to compare quality of foods.
Most people here on the board advise Ziwipeak or Acana as favourite high quality food, it has everything your dog needs but isn't packed full of fillers or nasty ingredients.
They may be more expensive that what you're feeding now but they're more nutritionally dense, so you only need to feed very little.
Purina Be Happy Dog Food | Review and Rating
Is this the one your using? 1 star rating, very controversial food there!