Hi Jawad,
Yes vWLC only support flex-connect APs and as we know 1850 don't support flex-connect mode.
Here is the proof: http://www.cisco.com/c/en/us/td/docs/wireless/controller/release/notes/crn81mr3.html#51900
Better to buy a small WLC like 2504 for each site.
Buy two WLC for each site:
Primary:AIR-CT2504-5-K9 --> with 5 AP license in it
Secondary: AIR-CT2504-HA-K9* --> HA model with 0 AP license.
I have many site with same scenario, as i dont like flexconnect mode.
Regards
Sandeep