I agree with MEL-Traveller about flying a day early. When flying to join a cruise I would definitely aim to be in Vancouver at least a day early in case of any delays as you never know what could happen. Look at QF9 a few days ago that had to return to PER after being in the air for a couple of hours because of an aggressive passenger. I haven't heard what time it then departed the next day but if anyone on that flight had needed to have arrived at LHR on time for something important that day they would have missed it.
A few years ago we were flying SYD-HNL on HA and staying there for a week before a cruise to sail from there back to SYD. Two hours into the flight we had to divert to BNE because a passenger had a medical problem. We arrived in HNL around 4 hours late and there were some passengers who were on a 7 night Hawaiian Islands cruise the same day and they should have just arrived in time but I know some of them were stressing about it.
Regarding booking award flights - you mentioned that the flight you need should be available on 17 Sep so I would check every day until then to see which flights are released that day and to confirm the time they become available so by the time the 17th comes you'll have an idea of the release patterns. Since 13 Aug 2019 should be released on 17 Sep, 12 Aug would be released on 16 Sep, 11 Aug on 15 Sep etc. I usually do dummy bookings regularly for a few weeks before the date I need to see that new flights are released each day as you would expect them to. When I was booking SYD-AMS on CX in J through QFF a few months ago for late May 2019 we had planned to leave SYD on a Thursday, a few days before a cruise we have booked. However since I saw, over a few months of checking, that at that stage CX were only releasing J SYD-HKG-AMS flights for Mon,Tue and Wed departures through QFF we booked the Wed flight. If we had not noticed that and had waited for the date that the Thu flight would be released we would have missed out.