Can an iPhone purchased in Japan be used in Canada?

You should not do that.

Your iPhone warranty will not be valid out of the country where purchased. Some iPhone models actually differ between these two countries. Apple may well not be able to service an iPhone purchased in a different country.

Can I buy an iPhone in Japan while on vacation and bring it back to Canada to use?

Probably, but you would be wise to contact the carrier in Canada to ask about this before you buy the phone. Get with your carrier in Canada and ask them what they need in the way of information.

Whether the warranty on the phone will be good in Canada is an open question, so you would want to contact Apple in Canada to ask them whether they will be able to service the phone from Japan and if so, will that be covered by the warranty.

The best advice that we can give has always been the same for many many years:

You should buy a phone in the country in which it will be used most of the time.

Re: “… You should buy a phone in the country in which it will be used most of the time …”

+1

Rock-Solid advice.