...First as the previous poster remarked the actual move of the option price can be dampened simply because the seller knows you cant force him to settle by exercising. I will concur that this is not so different from regular index options in European style though I would say that the VIX is more volatile (which sounds really weird to say

than the indexes are...
...More fundamental problem is the spread of VIX options during a big spike - they widen fast as sellers know that spikes are likely to average down quite quickly. So you may be on the right side of the deal but not be able to close your position until the VIX has returned closer to the mean. Again if you had American style options this could not occur in the same manner because you'd simply exercise. This effect is so pronounced that I have read in some books - like Jeff Augen or Kerry Given 's books - that its better to sell puts if you are bullish on the VIX because it grants a better ability to get out to make use of a spike. Both authors indicate that even then you should not expect to be able to get anywhere near the top end you would have with an American Style option.