Just give yourself a break and start drinking slowly. Very little Vodka over lots of ice drowned in lots of coke, lemonade, tonic, juice. Preferably use a flavored Vodka like Smirnoff/Absolut orange/mandarin, finlandia lime, cranberry..... If all Vodkas make you sick, it could be a psychological aversion so switch to Bacardi or Tequila for a while, drinking small quantities in the way suggested. Hope it works.