Can VyNDEX be used offline?

Yes. You can even use VyNDEX in an environment where an internet connection is unavailable, because VyNDEX is an Excel add-in.
Once you have loaded Salesforce data into an environment where an internet connection is available, you can work with the loaded data using Excel in the offline environment.

You can update the data that has been edited in the offline environment after connecting to internet.
* You may need to log in to Salesforce again.

(Updated on May 24th, 2016)