SAP Fiori Approval for VIM is already available.As some research, I find out 2 SAP Fiori Approval applications for VIM: The one provided by OpenText and an other one available on sapappcenter.

SAP Fiori Approval for VIM

SAP Fiori Approval for VIM has been available from Open Text Website since VIM 7.0 SP6 and VIM 7.5 SP2 ,

Check this link and you will also find the Approval Fiori for VIM.

You need a authorized account in order to navigate to SAP Opentext Knowledge.

For example, here is the link that you can use to download SP2 of VIM 7.5 that also includes a  folder for BC component to download required software.

SAP Fiori for VIM RoadMap

Actually, Open Text has launched for VIM based Invoice approval workflows their own Fiori app.
Currently this Fiori application is capable of approving Non PO based invoices.

If you are looking for both PO and Non PO based invoice approvals, you might need to wait for some more time.

Because Open Text has announced it’s road map already and existing Fiori app will be enhanced soon to incorporate PO based invoice approval scenrios and the latest app is expected to be released by the mid of 2016.

As standard SAP Fiori app, Open Text’s VIM Fiori app is also capable of extending UI and oData servcies as per the extension points.

So for your purpose, use this app and can make customisations on top of it.

Once the installations are done configuring this Firoi app in launchpad is just same as standard SAP Fiori application.*

*source: scn.sap.com

Vendor Invoice Management Approval App

The VIM App or Vendor Invoice Management Approval Application is used for processing incoming vendor invoices and making your life easy. The Fiori VIM application makes the approval process complete. 

This application allows you to approve, disapprove the invoice and also let you see the History and detail information. Next to that you can also view attachments on your mobile devices or desktop.

Check the full detail on SAP VIM Approval Fiori