I just updated avast Free to 12.3.2280 on Vista Home Premium SP2 and discovered that it fixed the recently-introduced problem of Windows Security Center not detecting avast but now "Windows Defender is turned off by Group Policy". That happened without my being given any choice about keeping it on.
From other forum posts, it appears that Windows Defender in Windows 10 has more functionality than the Vista version and running both might be inappropriate [and therefore either Avast or Windows turns Windows Defender off].
However, that is not the case for Vista.
Please rectify this problem.
Peter Jones