You can make a key that fits the table slot and mill a shallow groove
in the underside of the vise for it.

The RF-31's table slots had been cut at a slight angle to the X axis. I
fitted the key to the table first and milled its top edges slightly
narrower to make them parallel to the table motion. Then I clamped a
piece of ground stock to the table, indicated and adjusted it true,
clamped the vise onto it upside down and milled a slot to fit the key
very snugly.

jw