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This page describes the holes necessary to mount the Honda sissybar. Sadly, Honda no longer seems to make this item for the older VLXs, so your only chance is to find one used.
This photo shows the longer bolts I put in my seat mount, and the spacer necessary for proper fit. This this modification, the seat bolts are MUCH easier to work with.
Honda Shadow VLX Sissybar mounting hole details
This is a view of the rear seat mount. The sissybar holes are on either side of the hooks for the rear of the seat. It appears that it was necessary to use a hole saw to cut the fender, and then the metal frame underneath was drilled. I'd suggest using a nut and lockwasher on the sissbar bolt.
Side view of the left rear mounting hole. I don't know how that nasty gouge got into the fender. Yes, my bike is dirty.
Side view of the left rear mounting hole. I don't know how that nasty gouge got into the fender. Yes, my bike is dirty.
The front seat mount from above. I'm sorry this is out of focus. I forgot to turn the macro mode off. I'll try to re-shoot this image later.