How do i show the Notify button instead of Add To Cart button

Hi Mike this is automatic so long as this setting is on.
Go to your -product-default page and click on the Add To Cart Button addon, In the options for the addon on the right scroll down to options section "Stock" 

Now set the "Respect Stock Option" to ON

You can also define some more options here also.

When a customer uses the notify form, There details are added in the VirtueMart Product Details page under the tab "Product Status" added at the bottom of this tab page
See attached images