I do not detect any flavor differences in the two -- the only difference that I can describe is the texture of the two products. The granular isn't a powder (duh!) and this makes it a bit easier to...
It is certainly a different type of onion flavorwise, but it would definitely not ruin a recipe if you swapped it in. Depending on how it's used, it could even be an improvement.
Yes, the substitution can be made. However, it is important to be aware that vidalia onions are generally a sweeter and more mild onion and so you should start out with less of a red onion than you...