Heathrow Airport

Assuming this is one ticket and not two unconnected flights then the answer is no, you stay airside and do not cross the UK border.

This explains what you do:

https://www.heathrow.com/flight-connections

From ramblin' on's link:

Connection times: important information

It depends on your journey – your airline should quote a minimum
connection time, based on where you’re flying and whether you need to
change terminal. That will usually be between 60 minutes and 105
minutes.

If you're on two separate flights and thus two different itineraries between Vienna and Vancouver, 90 minutes won't work and you'll have to buy a ticket to Vancouver from Heathrow if you miss your transatlantic flight.

I'm not sure what you mean by one itinerary? The trip was booked all at once, travel to Europe and home.
It is on 2 separate flights, one from Vienna than change planes YVR.
As far as "something to declare", can you clarify? My trip includes Paris, Italy, Vienna and the plane change at Heathrow.
so if I purchase something in Paris, Italy or Vienna do I have "something to declare" at Heathrow? Or for that matter any of the other airports I land in. I will be flying from Paris, than after a stay on to Italy than after 2 weeks in Italy fly to Vienna, layover in Vienna than Vienna->Heathrow->Vancouver.

so if I purchase something in Paris, Italy or Vienna do I have "something to declare" at Heathrow?

no because they are all within the EU customs union.

Though depending on what you've bought you may have to declare purchases when you arrive at back at Vancouver

The trip was booked all at once,

You booked directly with the airline? Both flights, VIE-LHR, LHR-YVR are on the same airline?
Or did you book through something like expedia?

The big question is, is this one ticket or two?

If one ticket: GOOD. Your bags are checked through to Vancouver, you stay "airside" at Heathrow (no immigration), and you won't see your bags again until Vancouver, so no UK customs.

If two separate tickets: BAD. You will need to go through UK immigration (passport control), then pick up your bag(s), then carry your bags through customs, but no customs restrictions as you have come from another EU country, then exit through arrivals.
Then, double back to departures and check in for your second flight, hand in your bag and through security.
This will take a lot, lot longer.

I hope this answers your question.

You're probably confused by now but unfortunately it is not a simple question with a simple answer.

Who did you buy your tickets from?
What airlines are you flying?
Will you have checked luggage?

Look at your ticket or receipt. Are there two ticket numbers or two record locaters (usually a combination of six letters or numbers.)? If just one for both flights it's a connected ticket.

If two different airlines but within the same alliance chances are it's one ticket.

If seen as one ticket, then all you do is get off plane and connect to the next one. No immigration and no customs. You will go through security again.

If two tickets and you have luggage then you will have to go through everything. If two tickets but no checked luggage you could go to flight connections.

I'm guessing you're flying Austrian Airlines and Air Canada. If so, they are both in the same alliance so there is a good chance it's one ticket.

I'm not sure what you mean by one itinerary? The trip was booked all
at once, travel to Europe and home. It is on 2 separate flights, one
from Vienna than change planes YVR.

It is crucial that you DO know what I mean.

How did you buy your flights between Vancouver and Vienna? Online via one airline with one itinerary? Since your stopover is Heathrow, are you taking British Airways flights to/from Vienna? If not, what airlines are you flying?

One more thing, you wrote above that you have already booked your airline tickets. Why ask about stopover time at Heathrow AFTER you purchased your tickets and not beforehand?