You need idle population to train troops, its doesn't matter much having idle population available. What i suggest is not to build more cottages, just upgrade the ones you have and fill your city with barracks.
Maybe i misunderstood, you have 6550 population limit? But only 5106 people? I'll try to explain as good as i can. The "Vacant homes" is the percentage of your population limit.
10% tax, 5000 pop limit
5000 * .1 = 500
Total population will be 4500. (5000-500)
So basically if your tax is 0%, you will have full advantage of the population limit. The free spaces are just the tax percentage of your population limit.
Bah kind of early here, can't explain this better lol
Lynnore - Sever 6 - 1,91mil prestige.
Lynn - Server 49 - 1,01mil prestige.