>http://www.theregister.co.uk/2015/07/16/windows_10_will_update_whether_you_like_it_or_not_unless_you_have_enterprise_edition/
>
>This might warrant careful consideration if you have mission-critical legacy apps, such as those written in VFP.

Just make certain that you have a tested avenue (secondary boot, tested USB boot) to have the HW run an alternate OS so that your VM with the legacy apps is certain to run.
XP is good enough if you do not have to access the WAN even without an air gap.