Hello,
If I understood correctly, what you want to do is to set up your FreeRADIUS server so it consumes identity information from FreeIPA.
That is not the purpose of the radiusproxy functionality, which implements the reverse flow: clients contacting FreeIPA would be proxied to a RADIUS server for authentication. See [1] for a detailed explanation of a common use case for radiusproxy.
In your case, you need to configure FreeRADIUS so it connects to FreeIPA using LDAP. The authentication mechanism to do this could be username/password, or you could set up SASL GSSAPI, depending on your requirements. You may find this gist [2] useful.
Authentication may not be enough, though, and you may need to leverage other information (group membership, I would assume) in order to authorise users for VPN usage.This is done on the FreeRADIUS side.