I have a Ham-It-Up from NooElec and it works very well for upconversion. It should work equally well for downconversion but I haven’t actually tried it. It can’t do both at the same time, so you will need 2 for RX/TX.The frequency shift is always 125 MHz, so if the software can’t do the conversion, you can just do it yourself.
P.S. As I recall, the local oscillator on the Ham-It-Up isn’t that precise so there is an upgrade that improves it. For VLF, I would definitely get the upgrade. It doesn’t cost very much.