I wouldn't use ethnic restaurants as a mechanism to track immigration. In a multi-cultural society like the United States, ethnic food fads can "trend" without necessarily being indicative of the population. For example, a wild growth of fondue restaruants in the 70s did not indicate a massive swiss migration, nor did the proliferation of Sushi restaurants mean that Japanese were moving to towns all across the United States. Likewise, a sudden explosion of Pho restaraunts in a city does not mean that the Viet population doubled or tripled. It means that pho is more popular and the existing population can open more restraunts to meet the demand.

I would rely on census information first and foremorest. You can use ethnic restaurants in conjunction with other cultural activities (film festivals, parades, newspapers, churches) to make a statement about the result of an ethnic migration, but I would not use it alone.