You will not be allowed to board a flight to Canada without either a valid PR card or PRTD.
There will be no issues leaving Canada, you can leave with an expired card.
Do this:
1. Submit your application for the PR card, then travel.Your new PR card may not come before you leave - if it doesn't, do you have someone in Canada who can FedEx the card to you in Vietnam? Your roommate or partner or someone? If yes, then it's not a big deal because it's quite likely that you will get the card early in August. As long as someone can get the card and send it to you in Vietnam, you're ok.
2. Or, do not apply for the card and apply for a PRTD when you get to Vietnam. It's likely that you will get the PRTD in less than a month, but if you don't you will just have to wait till you get it. You will eventually get it, so there's no need to say "I don't know what to do". Again, the chances are good that you will get the PRTD in less than a month if you submit a complete application as soon as you arrive in Vietnam.
Or, you can choose not to travel at all.
Those are your only choices. No point worrying, you should understand your options and make a decision.