If an IPv4-only VPS hosts a VPN that assigns IPv4 and IPv6 addresses to clients, can IPv6 traffic be routed through IPv4?

This is a matter of routing, not of VPN.

You can tunnel IPv6 packets through IPv4 to their final destination that can be an IPv6 or IPv4 host. It is irrelevant if this happens in a VPN scenario or not.

Check some basic Debian how-to here and a practical example of IPv6 tunneling through IPv4 here.

The logic of it can be studied from a CISCO perspective also, which helps other general scenarios.

Make sure that when you implement this while using VPN all routing and network settings correspond to your needs and there should be no trouble.