Mercury is a Free and Open Source transport layer that uses OFDM to transmit digital messages over HF (same as VARA does). You might remember hearing of OFDM when we played around with Rattlegram two years ago... Because VARA is closed sourced, the way that RF layer of Mercury is not compatible with VARA. However, the API layer is.
Mercury is still experimental so it hasn't been integrated into Winlink yet, but hopefully soon.
Here's a good presentation with demo and Q&A by KM6LYW. It's over 42 minutes long so I recommend watching it at 1.5x speed.