VS measures over the bust and subtracts, a crazy method that results in many women having bras that are far too small and they size their bras strange too to ensure that women look like they're spilling out of them (and they know that most women incorrectly think that a C cup is average, which gets factored into their sizing). Use the under the bust measuring method or go to a place like Nordstrom that has a good reputation for bra fitting or Kohls where you can try on bras without the dressing room Nazis questioning you. I find the forms don't live up to the labelled sizes (cup sizes are relative to chest sizes, so a 34D and a 44D are quite different) so you may have to wear a smaller cup size bra than the forms are rated for or use extra padding.
Hugs,
DeeDee