If you don't want to use pbr package, then a set of rules/routes for the vpn connection is needed.
For a piece of advice, filter out the default gateway from the vpn client, so that you only have the default route from the ISP on the main routing table.
Then create a route for the default via the vpn gateway in a new routing table. Finally make a rule to match the traffic that will use the new routing table, that is to use the vpn.
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