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Easiest way would be to use borg create --read-special --chunker-params fixed,4194304 '/home/user/sdcardbackup::{now}' /dev/sdX
(which I copied from the examples in the documentation). I’m not sure if Vorta has a way to activate --read-special
but I suspect not; you can most likely still use it to make the repo and manage archives inside of it though.
Backing up from a command/stdin might also be relevant as an alternative, since that lets you back up more or less anything.
There’s been an exFAT driver in the kernel for a couple of years now (merged after Microsoft’s patent pact added ExFAT), it works fine. Same driver gets used on Android for SD card support.