[pushkins]

The "flanges" you are referring to sound like "Nailing Fins" if they have little holes/slots all the way around them that is what they are, they are for new build construction where you use the slots to nail the window into your framing.
Yes you can just cut them off and use the window as a replacement window with screws like you have mentioned.
Make sure you use a 100% silicon caulk for exterior use, it will bond with both the vinyl and the blocks.
Only use window and door expandable foam make sure it's not that "Great Stuff" it will expand too hard against the vinyl window frame.
Also make sure you use the foam last after the window is installed and screwed in place.