you shoul replace n with a zero based element number.
statements other than the first one are not real statements and are just examples. it is a syntax error to have v[n] as a statement.
what i wrote was C# code and i don't know if it works on javascript or not
It sounds like you're confusing two issues - how to access an array element, which is explained by Ashkan. If you have an array "myarray" of integers, then myarray[0] is the first integer. If it's an array of Vector3, then myarray[0] contains a Vector3.
What you can do with those vectors is another matter. If you go to the Script Reference part of the Unity manual and look at the documentation for Vector3, you'll see a list of operations, including vector subtraction, addition, etc. The Script Reference also has a section about arrays.