Do I need visa for transit at Xi'an Airport? (on the way from Vancouver Canada to Singapore)

Xi'an airport is offering an option of 24 and 144 hour visa free transit for a couple of years (in reality it was offering 72 hour visa free transit until now, and 144 hour visa free transit starts on December 1 of this year), so just don't listen to those who simply don't know why are they talking about something they simply don't know.

So, if your itinerary and your citizenship are eligible, you can enjoy 24 or 144 hour visa free transit in Xi'an.

By the way, you have mentioned an itinerary:

Vancouver - Xi'an - Singapore

This is a potential problem because there is no direct flight from Vancouver to Xi'an. So, if you are traveling to Xi'an via some other Chinese airport, you will be able to use only 24 hour visa free transit and 144 hour visa free transit is allowed only if Xi'an is your only stop within mainland China.