Please show me where you got that information - from what I know, that's not true, it is shown that nicotine clearly raises blood sugar levels. Here is one of just thousands of articles on the subject:
Nicotine induces prediabetes, likely contributes to high prevalence of heart disease in smokers
Just Google nicotine and Blood Sugar and you'll have an encyclopedia's worth of articles on the topic.
For those who don't see a change in blood glucose levels, note that everyone will have differences in how well their metabolism can adapt to the change. When waking up in the morning is probably when your nic level is the lowest, but the Dawn Phenomenon causes most diabetics to have a rise in glucose when they first wake up, so it would be hard to tell what the effect is unless you went to 0 nic for a day or two and then did some 24mg juice...
Sometimes other medications, such as anti-depressants, can lower the effects of nicotine absorption. Also, vaping nicotine is a slower absorption process, so there is probably less spiking than there would be with cigarettes. Also note that nicotine effects other things for diabetics other than blood sugar that are dangerous, such as constricting blood vessels...