If you want automate a periodic refresh of a VDB, you can use simply the refresh policies on the VDB (select "Manage" > "Policies" and select a policy for you vdb in the 4th column "VDB refresh".
Anyway, I'd like to make clear the terms used for dSources and VDBs, if you agree.
To make sure that your dSource is up to date with external linked DB, you have to snapshot it (you can schedule periodic snapshot with the snapshot policies). If you enable also LogSync you can get any "frame" of your DB between two snapshots and so your dSource is continuosly updated with your external linked DB.
To copy fresh data from the dSource into your VDB copy, you can do a refresh on VDB from the latest point of the timeflow of your dSource.
In few words, what you need is:
dSource -> snapshot
vDB -> refresh
And you can schedule periodic snapshots and periodic refresh through policies (Selecting "Manage" > "Policies" from the main menu).
Best Regards.
Gianpiero