Hi mama! Para sa specific vitamins like VITASEC, best approach is to ask your baby's pediatrician kung saan exactly makakabili, kasi sila ang magsasabi kung recommended talaga for your 11-day-old, at kung may specific brand or pharmacy ang clinic nyo.
You can also try major pharmacies like Mercury Drug, Watsons, or Rustans, call ahead lang to check if available. A few of my mum friends usually ask din sa hospital or birthing center kung saan nila nabili, since they know the trusted suppliers. Good luck, mama!