Installing a vinyl window
Beyond what was already posted, here are my recommendations based upon my current experiences with my contractors.
First, make sure your contractor understands and follows the window manufacturer window installation instructions. Failure to do so could result in loss of warranty.
Make sure appropriate window flashing is used over the sill of the rough opening and applied in an appropriate shingling fashion (over the sill first, then overlap that flashing on both sides going up the jack stud about 6").
Make sure contractor applies a 3/8" bead of caulk around nailing fin.
Make sure the contractor uses the appropriate nailer for installing the window (if they use a high pressure nail gun, they will almost assuredly crack the nailing fin, unless they adjust the pressure appropriately).
Make sure exterior nailing fin flashing is done in proper shingling fashion (sides first, then top over the sides, going beyond the window trim by 4").
Make sure the flashing is securely stuck down around all portions of the window, and there are no large noticeable bubbles/creases.
I am building a new home and, luckily for me, I'm doing a lot of the work myself. Due to this, I was able to notice the shortcuts the contractor took with my window installation (unfortunately, post installation and after 80+ % of the siding was put up). I experienced two leaks during a rain storm and know that I currently only have two types of windows in my home. Those that have leaked, and those that have yet to leak. I'm in the process of attempting to get my contractor to fix these, but I'll never be able to get the caulking issue resolved, unless the entire window is removed and replaced.
Watch what they do and make sure they do it right. A pretty nasty leak can go unnoticed for years, as insulation, drywall, and wood suck it up before you actually notice it.