You drove me to go to the website. You are correct. Koetsu lists the Vermilion as having an 0.2 millivolt output, much to my surprise. All the other Urushi variants are listed 0.4 millivolts. I can only conclude that something is different about the Vermilion. I don’t recall ever reading anything about it, but you probably do need to proceed on the assumption that the output is 0.2mV. In which case your present phono stage is marginal at best. But it is still not wise to insert an SUT before an MC phono stage.
I suggest you give it a try with your present set up, and then decide whether in fact you need a higher gain MC phono stage. I think Koetsu measures output at a stylus velocity of 3.54 cm/sec, the old standard. The new standard for many years is 5 cm/sec, which would give a higher stated output. You might luck out if Koetsu is understating the output by current standards. Like I said, my Urushi behaves as if its output is greater than the claimed 0.4mV.