Flexible duct is usually aluminum, and not allowed for cooking hood ducting. However, all is not lost -- there are steel duct fittings that have a rotary joint allowing one or two of them to achieve almost any offset. (An intermediate straight piece may be needed for larger offsets.)

See this thread for an illustration: https://www.doityourself.com/forum/ducting-air-circulation-ventilation-systems/544140-aligning-ductwork-exhaust-hood.html

Anyone has any clue on how to remove a old exhaust vent? This one seems to have no visible screws or anything. I have even checked above the ceiling and nothing seems to be holding it down but I cannot remove it.

It was installed prior to the ceiling being plastered, and my experience is these will not come down without removing the plaster ceiling around it. They are little tanks.

Ok so basically cut the ceiling around the vent and it should come out then?

Yes- typically they were nailed to the rafters/ joists with framing nails... This is the perfect opportunity to relocate the duct!