If you normally use your Twitter account to login Vine, just do the following.
Open the Vine app and go to Profile->Settings. Provide an email address and tap Reset password. Check your email for a link to reset your password,then follow through and set up an new password for your Vine account.
Now, come back to the VineGrab app, and login with your new Vine email/password credentials. We hope you enjoy VineGrab!