when they connect to the SW VPN I assume you are doing 1 of 2 things, either they are granted access to the LAN and getting their IP from your DHCP or they are put into a separate VPN LAN with a different IP scheme.
I have yet to find a way to trigger the scan, however you could put a scan agent in the vpn lan to continuously scan or you could create scan target lan on LS that targets there more frequently during business hours when they are likely to be connected.
the problem is this, you say a lot of people never connect to the home office, therefore they won't get scanned.
short of the expensive (putting LS in each office) you could use LSPush and schedule scans having them save the output to a shared folder at that office or in the cloud if accessible.
then import those as you get them or on some schedule?
will take a bit of engineering but I think it is doable.