Can the IFI Spike relays (http://www.ifirobotics.com/spike.shtml ) be used with the Vex Controller?

Yes, the Vex Controller is compatible with the IFI Spike Relay Module. The Spike control signals (PWM cable) must be connected to the Vex Controller Analog / Digital Outputs. The 3 wire PWM cable going to the Spike Relay Module must be modified because the Spike needs 2 control lines (inner and center) and 1 ground (outer) pin. The Analog / Digital Output 3-wire connection consists of 1 control (inner), 1 power (center), and 1 ground (outer) in a single row strip. Since the Spike expects a control signal on the outer white wire and the center red wire, the center red wire must be move to a digital output pin (wire colors based on our PWM cables). A direct connection without modification would connect the red center control wire to power and the Spike would not function properly. Also the sex of the PWM connector must be swapped from female to male which can be done using a 3 pin header. Call us if you are ordering a Spike relay and need a 3 pin M-M header.

I understand why the Spike can not be directly connected to a motor port. But could you explain how to connect to a digital output pin and how you would program that to control two solenoids.

See above post for connecting a Spike to the Vex Controller. For programming, you can program it just like you would any digital output. An "On" signal on one of the control lines and an “OFF” on the other control line would cause the Spike to actuate in one direction, while reversing both control signals would cause it to actuate in the other direction. You can use the MPLAB IDE or easyC software to change your program, but the User must know how to program.