You should be able to Undo the effect when applied from the Nyquist Prompt, but it won’t be listed as “vibrato”, it should be listed as “Nyquist Prompt”.
If it does not Undo, then either you have applied the effect more than once and you are only Undoing the last application of the effect (press Undo again to Undo the previous action, or go to “View Menu > History” to undo multiple levels in one go), or there is something wrong, in which case please specify which version of Audacity you are using. Please could you check this, as the Undo should work with the Nyquist Prompt.
I deliberately set the values quite high so that the user (you) could hear easily what it is doing.
If you want to customise the script and set your own default values, then see here for the format of the ;control settings: Audacity and Nyquist
You will see that the last three parameters are initial-value minimum maximum
The initial-value is the default when you first use the effect in an Audacity session. Set that to whatever you want.
Unfortunately Nyquist plug-ins do not support a Preview function, though I have it on a wish-list here: Wish List - Preview button for 'process' plug-ins