I'm interested in measuring the velocity factor of some coax I have (more of
an exercise than necessity). In order to do this, the MFJ-259 Operations
manual states that the "stub" to be measured should be attached with a
50-ohm noninductive resistor

Hi Jason, I just looked at the manual for the 259 and the 259B and it did not
mention a 50 ohm resistor in the velocity factor section. With both
instruments you measure the electrical length of the coax and compare to the
physical length to obtain velocity factor. It is a bit easier with a 259B
using the "distance to fault" function. The coax can be open or shorted (far
end) during measurement. No resistor needed as far as I can tell.
73 Gary N4AST