After a recent network upgrade, we want to be sure that VPN traffic is being directed to the highest capacity circuit

Current Sophos hardware: SG210, firmware 9.413-4

Prior to a week ago, there was one broadband WAN interface connected to the SG210; speed 120Mb up/15 Mb down.  An additional fiber optic WAN interface was added: speed: 100Mb up/100Mb down. These two WAN interfaces are configured to load balance.  Several remote users use VPN tunnels to access resources within the home office LAN using Sophos' VPN client software.

Is there any way to "steer" the VPN traffic to use the fiber optic circuit in preference to the broadband circuit?

This question came up because one of the remote offices recently upgraded their connection to a broadband circuit ( 50 Mb up/6 Mb down) and saw no difference in the performance of a business critical accounting application which is hosted here in the home office.  Clearly, this means that the "bottleneck" is somewhere else.  I was just hoping to "tweak" the Sophos configuration (if possible) so that the Sophos appliance could be eliminated as being part of the problem.