Vita sand entices reptiles to eat it, and it doesn't dissolve in the body. It will cause impaction. Give the gecko time to adjust to the change in environment. Wax worms aren't good feeder insects and shouldn't be given ever; they are very fatty and addictive to geckos.
If the food is being thrown up or passed undigested, check your temperatures, as they may not be warm enough. Use a digital thermometer with a probe to accurately read the surface temperatures. Make sure they're right around 90F