I spent awhile figure this about but eventually found someone that knew how to do it. From what I understand, you already have the VPNs themself working so they only thing left for your to do is to have specific client going through specific VPNs. To do this you need to have what you consider a default VPN providing internet to everything first. I usually restart VPNs until this is working correctly and it seems to continue working but it may also happen to do with the fact my Outbound NAT has my default VPN rules above the other VPNs (i'm not exactly an expert on this).
The next thing you need to do is to put a static address DHCP address on the clients you don't want to be using the default VPN. This is done at status -> DHCP leases.
Finally, you need to create a firewall rule that that forces those static address through those alternate VPNs and place them above your rule that normally allows clients to get internet. If your static dhcp address for that client is 188.132.1.3 then the rule looks like:
Interface: LAN
Source: 188.132.1.3 (using single host or alias)
Destination: any
Gateway:<the name="" of="" your="" selected="" vpn=""></the>