I think you should be more specific as to what Vs and Vss are. The way H-Bridge motor driver ICs work is you supply a DC voltage to the board for motor power (Spec is 6-35Volts) and it will use that to drive the motors. On the L298 that is the green insert at the bottom near the pins. You plug each motor into its respective green insert.
The GND and 5V pins are regulated output pins if you want to use them (assuming you use the recommended configuration http://www.canakit.com/Media/Manuals/UK1122.pdf
IN1 or 2 are used to control the direction of motor A.
IN3 and 4 for motor B. *I am not sure why there are two direction pins for each motor... They're digital. Low and it'll turn one way, High and it'll turn the other.*
The speed of motor A or B is controlled by a PWM signal to ENA or ENB respectively. The greater the duty cycle, the faster the motor runs.
*EDIT: Okay, so I remember why there are two. To drive a motor in one direction, one is held High and the other held Low. To reverse direction, the state of each line must be flipped.
Hope that helps!