First: the widely cited number of 25,502 empty homes in 2016 is not correct. That's the number of private dwellings not occupied by usual residents in the City of Vancouver. Some of those are occupied by people who make their usual homes elsewhere (either in Canada or abroad). Whatever problems you might have with that, they're not all empty. 20,000 might be a better estimate for empty units if we extrapolate from 2011 census data, and it's worth noting that there will always be some empty units due to turnover etc.
To put this number into perspective, 20,000 homes is less than a year's worth of housing starts in the region, or a few years of development in Vancouver. It's a good start, but it's nowhere near enough to solve our housing crisis, so putting building homes on hold while we fill those houses would be counterproductive.
In the long term, a more permissive housing policy would mean that we have enough homes for long-term residents and people who are willing to pay property taxes without consuming the usual amount of services.
Sometimes, estimates of the number unoccupied units are conflated with the purpose built rental vacancy rate, a statistic published by the Canada Mortgage Housing Corporation that measures the amount of slack in the rental housing market specifically. This number is very small (less than 1%), meaning that prospective tenants have few choices while landlords have many choices, which puts upward pressure on rents that should be alleviated by increasing the pace of rental housing construction.