VPN is not a viable option for using an Access app over the web. It will be too slow but Citrix and RDP are excellent options and are unlikely to require any application changes.
I think that need to be explained. While using a VPN to connect a local client running an Access front end to a back end on a server would not be ideal, there is nothing inherently wrong with using a VPN.
Connecting to a server with RDP via a VPN works fine. What matters is keeping the front and back ends on the same LAN.