Your flight to LHR is highly likely to be running late, as it’s probably the last flight of the aircraft that day. Chances are high delays will have accumulated during the day, and will manifest itself by being late inbound to VIE.
Once you arrive at LHR, better hope they’ve got their stand allocation and staffing up to speed. Otherwise you might have to wait for a stand to be vacated, and that can easily add 15-30 minutes. And if they do have a stand ready, better hope there’s someone to drive the airbridge
Then there’s security, where the UK for some bizarre reason have decided to run all connecting passengers through security - the same passengers who’ve just gone through security at another airport. But, that’s besides the point. There’s a chance Heathrow will have sufficient staff in place by October, and they might even have opened fast track too, and luckily for you it’s rather late in the evening. Still, it’s not unusual having to wait 15-30 minutes to pass security
You also have transfer bags. This is Heathrow we’re talking about, the place where transfer bags are suffering a high chance of not making a connection when there’s 2 or more hours between flights.
All in all: Chances are very high you’ll loose your connection unless everything is on time, stand and staff are ready and in place, and there is no queuing for security. And if you do make your connection, your bags probably won’t.
Why anyone would voluntarily book a connecting itinerary via Heathrow is a mystery to me.