If your cover (the vegetation) ends at 13 feet more or less, the structure (point, ridge, hump, channel, etc) typically continues beyond that and bass do use that structure beyond the cover. Not to belabor the point, but structure consists of the different projections, humps, etc that form the shape of the bottom of the lake. Cover consists of the trees, stumps, vegetation, docks, etc that are on the structure. Breaklines are lines formed by a sharp change in depth, outside line of vegetation, change in bottom substrate, etc. You might want to research these in depth as it will help you in your search for bass and I am only providing a cursory explanation in this post.
Your vegetation will end where the turbidity of the water and the depth no longer allow enough light for it to grow. If your lake is deeper than where the vegetation ends, bass will use a structure that extends beyond that depth. As for exceptions to the rule, if bass are not the apex predator and some other organism exists that will feed upon them, bass will be less likely to use the part of the structure that is in open water with no protective cover. Also, during the summer when a thermocline develops if it forms at a shallow depth bass will not go deeper than that depth because there isn't enough oxygen to support them.
It is likely you are missing out on some good fishing if you don't fish beyond the vegetation, but then again if your lake bottoms out at 17 feet you may be fishing most of it anyway.