Yea as far as I know the ninja regs such as the slp, smp, shp are fixed pressures and cannot be changed. I've yet to hear of anyone adding a shim to them.
Don't quote me on it but I think the input pressure has to be at least double of what the operating pressure of the marker is for it to cycle properly. In the case of the vanquish, it operates between 180-200 psi so slp would probably be too low. Perhaps with some breaking in, one might be able to lower their operating pressure but I'd personally stay at 450 and up. I could be wrong and the vanquish could possibly cycle fine with an slp. If he wants to try it I say go for it.