It seems safe to assume that the same percentages apply to photographers in general, including travel photographers, and those on vacation.
Unfortunately, many photographers who wear glasses seem to have difficulty focusing their DSLRs and composing their images due to their inability to see the image fully in their camera's viewfinder, “edge to edge.”
Fortunately, for most photographers who wear glasses, there are both equipment solutions and techniques to help with focusing and image composition.
We need to start with the photographer's glasses. Prescription lens glasses are either single-focus or multiple-focus. Multiple-focus lenses can be bifocals, trifocals, and progressive lenses. Of these, the most difficult to use while photographing are trifocals and progressive lenses.
When looking through the viewfinder, a photographer's vision must be the same, “consistently,” so that corrected, every time you look through the viewfinder, the focus screen and viewfinder display will be in sharp focus.
Since multiple-focus glasses, have small areas of discrete focus on the lenses, it is essential that photographers look through the same area of the lens each time they look into the camera's viewfinder, so that they will have the “consistency of vision” necessary to produce sharp, well composed images.
With a “consistency of vision” the DSLR's “diopter adjustment” can be properly configured to allow each photographer to see the focus screen with its AF (auto focus) and grid markings, and viewfinder display in sharp focus.
Looking through the viewfinder, you adjust the “diopter adjustment” via its control until the viewfinder display, focus screen's focus points, and AF area brackets (if any) are in sharp focus. Once that's complete, an image on the focus screen will display the image's focus accurately.
According to your vision, the camera's built-in “diopter adjustment” may be inadequate. If it is, many camera manufacturers, such as Nikon, sell “correction eyepieces” which attach to the viewfinder. They are available in both “+” and “-” corrections. Coupled with the “diopter adjustment,” and your glasses, most vision correction can be accommodated.
Many photographers who wear glasses add an eye-cup, with or without magnification, on their viewfinder to enhance looking through it. If the viewfinder eyepiece is hard plastic instead of rubber, and doesn't have some sort of rubber bumper, an eye-cup is an especially good idea, as it will prevent your glasses from being scratched.
Even taking into account the above, getting your images in sharp focus and getting solid composition is not a slam dunk. Understanding your camera's features which can assist manual focus, and understanding how auto focus works, is critical to good focus.
Auto focus, in DSLRs, generally works passively, not by emitting a signal to determine distance, using contrast detection or phase detection. Both these methods rely on contrast for achieving accurate auto focus.
For stationary subjects, for example, (I'll discuss focusing on moving subjects another time.) if you can, focus on an area of your subject by selecting a focus point which corresponds to a sharp edge or pronounced texture. If you're using a single focus point, make sure it's one which is a cross-type sensor which uses two-dimensional contrast detection, as it gives higher focus accuracy.
This may mean that you need to first focus, and lock it in, recompose your image, then make the photo. Frankly, it's often the case that you can't expect to achieve sharp focus by composing your image and immediately pressing your shutter release.
Note: There are a number of reasons you may not achieve a sharp focus quickly enough; low contrast subject, slow focusing lens, previous focus setting far from new setting. To eliminate that problem, prefocus your camera/lens before making your photo.Most DSLRs have a “focus indicator” in their viewfinder displays, which can be of enormous assistance when you are manually focusing, especially for those who wear glasses, and whose vision is corrected.
I recommend the use of the “focus indicator,” even for those with excellent eyesight to at least confirm your focus is in the right range when focusing manually.
Also, if you're having trouble getting your focus dead-on at times, you can increase your depth of field to give yourself extra latitude in focusing, to allow your focus to be off a bit.
Many who wear glasses can't see the image in the viewfinder edge to edge, which makes composing each image accurately, a little more difficult. This occurs because when you wear glasses, your eye is not right at the viewfinder eyepiece, but instead back a bit, behind your glasses' lens.
You can compensate for this by literally looking around your viewfinder, by moving your eye behind your glasses up and down and side to side to see the edges, while keeping your glasses steady to maintain your “consistency of vision.”