is to initialize vector with a initial capacity, and capIncrement parameter mentions by how much should the current capacity be increased upon filling the vector.
Think of vector as an Array. Instead of increasing the size of vector at every insert into the vector, providing init capacity makes lot of sense as you will avoid un-necessary array copying and re-inserting
Why are you using Vector in the first place, rather than ArrayList?
A lot of people think it was a mistake to permit users to set a size increment like that. So ArrayList doesn't have such a constructor.