What city are you seeing the number of vacant homes outnumbering homeless people by chance? Not disagreeing with you just curious as real estate is localized. Rents are high because after the housing crush, supply got absorbed by REITs and wall street, prices bounced back and went higher. Americans credit was ruined in the last economy crash so no one can get a loan thus they rent. Cities wanted more tax revenue so they value the properties higher, higher property taxes equals higher rents.

The issue you bring up with homeless vs number of vacant homes though goes deeper into the fact that even if you were to put every homeless person in a vacant house, who would manage the situation, pay the bills, keep the lights on, etc. Most homeless while they are not on the streets by choice have some sort of issue that keeps them from holding down a job, keeping up with their bills, etc. so putting a homeless person in a home is like putting a band aid on a broken bone.