Start with the 5/50/500 rule:
Do at least 1 trouble-free 5 mile ride before you try a 50 mile ride.
Do at least 1 trouble-free 50 mile ride before you try a 500 mile ride.
Do at least 1 trouble-free 500 mile ride before you consider the bike to have all its glitches worked out.
Then there's the Syscrush rule:
Make sure you've got your cell charged and some room on your credit card.
A buddy of mine did a Toronto-Calgary-Toronto round trip 2-up on a CB750 about 5 years ago. If they're set up right and the conditions aren't too troubling you can definitely use these bikes for long trips. It wouldn't hurt to call a good shop in Vancouver to look the bike over for you while you're at your friend's wedding and make sure everything looks OK before you head back.
Winnipeg-Vancouver is about 2300km or so if you go by the most direct route. It's definitely possible for your bike to go there and back without incident, but on a trip that long you'll encounter a range of weather and road conditions that will stress you and your bike, including:
1) Extreme heat.
2) Rough road surfaces (vibration can cause/uncover a variety of problems).
3) Very steep uphill and downhill (you'll be crossing the f'n Rockies).
4) Large elevation changes (see above).
5) Rain (can mess with brakes, chain, electrical).
6) Cold.
The one change I would strongly suggest before doing this trip is getting a voltmeter on there. Your charging system doesn't have to be really wrong to leave you stranded, and that's not limited to these old bikes (don't ask me how I know). I am a very big fan of
this one. It's just 1 LED, easy to mount and install, and it tells you a ton of info.
Another thing to consider is putting some kind of crash protection on the bike (like engine guards) at least for the trip. If you have a gravel-induced low-speed low-side, it's nice to be able to stand it back up and ride to the next town for repairs rather than having to call a tow.
Sounds like a fun adventure. Let us know how it goes!