If I remember right, the ring around the finder eye-piece of my VT Deluxe came off. You could take a pair of small pliers, cover their jaws with something which won't scratch metal, and try gently to unscrew the ring. Gently.
The correction lens for the F-1N may be of a different diameter, its threads may be wrong...
The prescription of your reading glasses will probably not work. What you want is a correcting lens which lets you see clearly, through the finder and with your glasses off, things which are between a yard and two yards away.
My solution was to have a pair of glasses made in which the right lens is used for the finder and the left is a shade more positive than my reading prescription so that I can read the engraving on dials and so on. When not being used, the glasses hang around my neck.
[Edit] The right lens is necessary when I use one of my SLRs but is not needed with my IIIc, which has enough correction built in. The left lens is necessary with all film cameras and works also with the LCD of my digicam.
[Second Edit] Oops. The yard or two is for SLRs' focussing screens. I see that you have +0.50 on your Leica, whose finder is different from those of SLRs but which presumably needs the same correction as that of the Canon 7. Go gentle with the pliers...
