>> Every vacant home is essentially a theft from someone else who has no home or is priced out of the market.

If you are correct, then the thief is not the private owner. It is the government that refuses to make it easier to build more houses.

Well, I don't know which "solution" is more utopian. Land is not infinite.

Greater Los Angeles has a lower population density than Germany or the UK. It's densely populated for a country, not so much for a city. The County of Los Angeles is somewhat denser at about twice the population density of England. Los Angeles itself is fairly densely populated, at slightly below the population density of London, or 1/4th that of Paris (a city known for resisting skyscrapers).

Allowing more homes to build does not require infinite land.

How do you differentiate between a landlord of an empty house, and a speculator with an empty house?

Attempting to would introduce an unnecessary loophole, as the stated aim would seem to include speculators with empty houses.

I'm specifically saying don't distinguish. The public should strongly incentivize against leaving houses vacant, whether you're a landlord without a tenant, a speculator with an empty house, or some rich dude with 50 houses he visits for a week per year.