This isn't going to be what you want to hear, but the viewfinder in my VI-T (and my P) improved dramatically after I had cataract surgery :-/
Having said that... you probably already have the PDF of that "Model VI T, VI L, P Service Manual" (actually just a collection of exploded diagrams and a parts list) that floats around the Internet, but having scrutinized that plus my reference photos of my own VI-T's innards, I don't think there
is any glass between the 3-position thingie (called the "magnifying lens box" in the parts list) and the frameline/parallax compensating plate, and behind the frameline plate there's nothing except the eyepiece... so I figure it's
got to be one of those.
It may not look like it, and it may be super-tight, but the eyepiece just unscrews. A flat rubber sheet will get a good grip on it, and so will the smallest size in that "Sucker Opener" kit from Japan Hobby Tool. Once it's out, you can have a good look at it and make sure the haze isn't on the inside surface (mine was hazier than I had expected.) The threads are very fine, and the rim is soft aluminum, so getting it back in afterward is a slightly ticklish operation, but all it takes is patience.
Incidental fun fact: According to that parts list, Canon offered these eyepieces in five different diopters (and eyeglasses wearers such as self really wish those could be found today!) Irritatingly, although they all had different part numbers, the the eyepieces don't seem to have been marked in any way (if someone knows differently, would love to hear it!) so if you have two VI-series Canons and they seem to fit your eyesight somewhat differently, it could be that someone went to the bother of ordering a different eyepiece back in 1959...
Anyway, next forward from the eyepiece is the frameline plate ("parallax compensation plate" in the parts manual) and while this is a good candidate for getting gunked up (you can examine it easily with a magnifier once the eyepiece is removed) I would be
very leery of trying to clean it. The silvering for the framelines is on the side of the plate that faces the eyepiece (I think; anybody know different?) so it's not hard to clean that side, but the silvering is EXTREMELY fragile and there are already enough VI cameras with eroded, faint, or completely erased framelines that you wouldn't want to increase the worldwide supply!
I suspect some of the frameline deterioration may have been just due to the passage of time, but I suspect a lot of it has been through ill-advised cleaning; I can easily imagine Ted Thriftshop back in the late 1990s saying, "Heck, I'll just squirt some lighter fluid on a Q-tip and it'll clean that sucker up in no time!" with disaster subsequently ensuing. Incidentally, while the frameline plate looks and feels like glass, I have read (on the infallible Internet) that it's actually plastic -- anybody know for sure? -- which would mean a lot of common cleaning agents might damage it. So there's that.
Well, that's as far as you can get without removing the top cover, but if you do that, you can clean the other side of the frameline plate, and then you can get a look at the 3-position thingie, also known in the parts list as the magnifying lens box. This is another good candidate for collecting gunk, since there are eight glass surfaces in there... you can clean the exterior ones easily enough by rotating the box to its different positions, but getting at the inside surfaces looked to me like a hard project since the lenses seemed glued into place. Still, if you can get the reachable surfaces cleaned, that most likely would help quite a bit. I've left mine alone so far, other than looking at them when I had the top cover off, but mine is pretty good other than the eroded framelines.
Yes, the V series optics are probably less problematic (and the ones with gold coating are probably less problematic than the ones with silver coating) but I still like the VI better because I like having parallax-compensating framelines...
