Note: “Next Server” field does not accept more than 1 IP address and option 66 has dependency on this “Next Server” setting. LB VIP should still be setup to load balance TFTP.
Follow steps mentioned in below article to configure the infoblox DHCP appliance
Next Server: Enter the IP address or hostname of the boot file server where the boot file is stored. Complete this field if the hosts in your network send requests for the IP address of the boot server. If the TFTP server is the NIOS appliance that is also serving DHCP, enter the IP address of the appliance.