Yes, this is my question also. The VPT is great for dries, but doesn't cast well in close - it needs some line out to load. I didn't look at the taper design for the Amplitude Smooth Trout, but I presume it's SA's standard WF Trout taper just in the Amplitude line. So as mentioned that should work fine too, though the VPT is (really was) more of a dry fly line. The VPT is no longer made but I presume by your question you tracked one down.Define "medium"...
I thought VPT would be discontinued, and I thought GPX/MPX would be too with Infinity being released...but both stayed right there is the inventory.....I doubt they currently sell much of either. If you have a really light, moderate action rod, like a 2-3 wt IM6/WT, Scott G,G2, GS 773, Orvis Superfine 3wt, etc. The VPT will load in close and deliver dries very delicately....if the rod has backbone or is heavier 4wt or 5wt rod....VPT becomes a longer distance dry line....IE: VPT works great for rods that load with less than a standard head weight out of the guides....otherwise it's a delicate long front taper true to weight line better with some line out. SA Trout taper is much longer rear taper and more progressive throughout the head length...similar to Expert Distance Taper, it enhances turn-over with a heavier back end.I stand corrected. I got some on discount and so had assumed they were being discontinued, but obviously not the case. And yes it is a finicky line that doesn't work with all rods.
HelloThe VPT is still in production and still a staple in SA's Mastery series:
Mastery VPT Fly Line | Scientific Anglers
It's a great line for the right rod, but the rod must be delicate. My question would not only be what is "medium" but even more importantly...for which rod? Line in and of itself is just line - with various tapers and weights - no one line matches all rods. I have opined at great length - Let the rod pick it's line for it's specialty purpose at hand. So to answer that question, I need to ask some questions:
For which rod, and, for fishing where in what habitat, with which technique and flies?