If you're talking about some class template named Vector that you have created yourself or gotten from a third party library then it is difficult to say for certain without knowing how it's defined but my guess is that it represents a mathematical/physics vector and 2 probably means you want it to be in two dimensions (i.e. contains two numbers).
Learn C++ with the standard library first.
Understand the Lattice-Bolzmann method (yuk! there are better methods for CFD!) second.
Then play around with the L-B examples (making sure that you can actually compile them).
I believe that
will, storage-wise, behave like
However, this OLB version probably has quite a lot of convenient geometry-associated methods defined for it, making it behave like a mathematical/geometric vector.