Can you really narrow the range of thought by narrowing vocabulary, as the Party seeks to do? What does that mean for people with small vocabulary- would they think less?

The idea is that if people simply cannot express themselves through words, over generations, they will cease to feel the things they cannot express. I don't think you can transfer this idea to people who simply have a low vocabulary. These people can still feel intensely and express themselves in other ways than with adjectives or verbs. The Party wants to destroy individual emotion and Newspeak is just part of their plan. They use fear, indoctrination, and propaganda in conjunction with Newspeak to achieve their ends.