It is just hard to not get fingerprints on the tape.
Stick the tape onto some waxed paper first. Then cut out the waxed paper and tape together. Don't peel off the waxed paper until you're ready to do the final assembly. This reduces the risk of getting fingerprints onto the sticky side of the tape, and makes it easier to handle.
Cindy, as to the vinyl repair tape being too thick -- it might be, depending on how much "feel" you need to retain from the buttons. It's the sort of thing that requires a bit of experimentation.
Yet another thought, for people who don't want to cover up the whole thing with tape but are afraid of the silicone getting out of control: Sometimes at electronics stores you can find stick-on "feet" made for applying to the bottom of a disk drive enclosure (some disk drives come with a sheet of them.) Picture-frame "bumpers" from a framing-supply store are similar, but larger; you'd need to trim these down with an art knife. Whichever you use, try cutting to size and sticking onto the buttons. They get knocked off fairly easily, but when that happens, just stick on another one.
Note that these do raise the button considerably... but that might make it easier to locate and press. It might be a good way to make the focus buttons easier to find and press when you're holding the camera up to your eye.