Seedvault is generally not enabled in secondary users. It is possible to use adb to enable seedvault in secondary users.
To find the numeric ID (eg. 10) of the user profile currently in focus use -
adb shell am get-current-user
Use this number in the place of [ID] in the following commands
adb shell bmgr --user [ID] activate true
adb shell bmgr --user [ID] transport com.stevesoltys.seedvault.transport.ConfigurableBackupTransport
You can check that the commands were successful using-
adb shell bmgr --user [ID] list transports
Seedvault can then be setup as usual in the secondary profile.
It is possible manually activate the Seedvault restore activity.
You can do this using the Activity Launcher app, or when connected by adb it is possible to use -
adb shell am start-activity -a com.stevesoltys.seedvault.RESTORE_BACKUP