If this a o/w ticket you can skip the last leg; no issue. No need to cancel; it would be nice if you ask staff at VIE to unload you from the flight to IST, i.e. you will no longer be checked-in for that flight and gate staff will not be paging you before closing the gate.

Check-in staff at LHR is unlikely to allow you to short-check bags to VIE only. So you need to travel HBO. In any event you can always ask upon arrival VIE for your bags to be delivered to you on the luggage carousels in VIE; your explanation: you don't feel well and are unable to travel. You bags can't be loaded on the flight to IST when you are not boarding that flight. It may take a while for the bag(s) to be delivered to you at the carousels.