When considering replacement windows, first look at your house and type of construction. If you presently have steel or aluminum windows, vinyl replacements will work and look good.
If your house has wood windows, vinyl replacements will look just like they are-cheap. I have seen vinyl replacements (which should have been wood), installed by large window companies which were held in place with caulk. How long do you think this will last ?
Pella, Marvin and Andersen all have wood replacements that slide easily into existing openings where the original window frame is in good shape.