Can a united airlines travel voucher be used on partner airlines? Example United Airlines/Air Canada
You can use it to buy a ticket from United. Under certain situations, known as a code share, United will sell you a ticket on an Air Canada plane. They generally won't sell you a ticket from one Canadian city to another unless it's part of a longer itinerary, and Air Canada won't operate a plane from one US city to another, but if you want to go between say Toronto and Chicago, and you can find the flight on the United website, you can buy it. Generally it will be marked as a code share but you can always do a little searching on the Air Canada site to find their flights and figure it out yourself.
Note you will still have a United ticket - this means that you are unlikely to be able to upgrade, for example, using Air Canada upgrade instruments etc.
You must book the ticket through United. However any flight partner that can be booked through United is valid. Your specific situation applies to me. I had enough miles on United to book an international flight. I found a flight on United.com that went through Air Canada. They took my miles without a problem and I was able to fly on Air Canada with United miles. The only hiccup is that you can't really pick your flight seating ahead of time unless you call Air Canada directly about the situation.