Here's my roundabout way of doing it with macro's, there may well be a better way but this works:
* Create an instrument rack. It's one of the default live devices.
* Add the synth you're using to the rack (drag it into the marked space on the rack, anything will work, live device, vst whatever)
* Expand the panel for your instrument by clicking the right-facing arrow at the top left of it. Click configure, then click whatever changes the sync on the vst's gui, then click configure again. You should have just created a slider in the instrument panel with something like LFOSYNC as the name.
* Open up the macro panel on the rack (just below the bypass/power icon at top left, its like a little circle with a bent in bit). Click Map Mode, click the slider you just created, then click the Map button under Macro1. Deselect Map Mode.
* Check that the first knob in the Macro Panel adjusts your sync rate. If not, you did it wrong, start again
* Right click the knob, and click edit midi map. Press the button on your controller you wish to use for changing the speed. In the window that just opened up to the left of your arrangement/session called "Midi Mappings", when you press the button an entry gets added here - the min and max values will be the values switched between. The only real way to get the numbers except by guesswork is to, BEFORE doing the "map mode" part of this, drag the slider you created in the instrument panel around and watch the GUI of your synth change, and make a note of the number under the slider when the rate is what you want.
Voila, a button that switches between 2 rates. You can make it much more complex than this, but its a start. Sorry if it dosen't make a lot of sense, I realised halfway through it's more of a bitch to explain than i thought
D'oh, this is for ableton live btw, forgot to mention.