Most of the vinyl windows with built in nailing flanges make the flange as part of the extrusion. Replacement vinyl windows are normally made without the flange. They fasten to the jambs with four screws, one in each corner, on the vertical sections only. If you look at each side where the sashes travel (if this is a double hung) you may see a removable cap top and bottom, about 4" tall and an inch wide. Pop it out and there should be a pre-drilled hole to run the screw through. You may not have the same setup on your window, but you get the idea on how to mount them to the jambs. If you go to your local big box store, take a look at their replacement vinyl window display and you will see what I am talking about. Should be simple to adapt over to your window.