Is it expensive to travel to Vietnam?  

I hope you will visit one day. The plane ticket is probably the most expensive. Could range from $1,000+ depending on the month. If you do go, I recommend going after Tet holiday like in April or sometime in November-December since many places are closed during Tet. Living costs, eating and drinking will be cheap but you can splurge and use up a lot of money fairly quickly. Most average meals are like $2-3 in the city. Outside the city, like in rural places can cost less than a dollar per meal.

Round trip plane tickets from California to Vietnam are like $1500-$3000. I don't know about Canada.

But once you get there, generally speaking, you can live on $700 a month pretty comfortably. Locals live on less than that since the average salary there is like 5 million VND which is $215. You can find street food less than a dollar but if you eat at an international food chain like McDonalds, expect to pay international prices $3-5 a burger. I like traveling to Vietnam because you have options to eat really cheap (less than a dollar) or really expensive ($100 bowl of pho).