How does spread spectrum VCXO (voltage controlled oscillator improve EMI on your board?
Spread spectrum improves EMI by spreading out the peak emission. Instead of one very strong spike at a high energy level, you end up with a wider "peak". The power output is the same, at the cost of frequency accuracy.

The benefit to this becomes apparent when you have specific emissions requirements to meet. You may not be permitted to emit more than xmW at a given frequency. Without a spread spectrum clock, you might not come close to meeting this requirement, but if you spread the emission frequency out a little, the power at any specific frequency in that range might fall below the maximum allowed, and now you pass.

The downside to spread spectrum clocking is that your frequency is no longer precise, because it (intentionally) wanders over a wider range of frequencies that are centred around your desired frequency. The average frequency is your desired frequency, but at any given point in time you will somewhere in the spreading range. This can cause trouble if you're trying to communicate with other devices.

Here's an exaggerated picture of the difference between a clock output at a specific frequency vs the same clock output with some spreading. Ignore the "noise level" note, or instead imagine that the dashed line "noise level" is the absolute maximum level that you're allowed to radiate in order to pass emissions testing)



(The picture source is http://www.tapr.org/images/ssfig1.gif.)