The exact same thing happened to my first two plants of V. denisoniana. In both cases it started a few weeks after they arrived in April and my guess is that the plants, which were not seed grown in the US, were used to warmer conditions. I finally got hold of a denisoniana grown from seed in the US and it is a very strong grower that doesn't show any of these tendencies. You should be able to save your plant I think but but you must be quick and you should try something more potent than neem oil. Try Thiomil or Cleary's 3336. Other than that, just follow Zain's advice.