Address this as a problem with a solution.
Problem - there was a gap in knowledge while you were away. It's not your fault for not explaining absolutely everything you know about the situation - that's impossible. It's not your boss's fault for not knowing how to do your job perfectly while you're gone. This was a simple gap, with potentially big consequences.
If you avoid pointing fingers, you stand a much better chance of keeping finger pointing out of the situation entirely. If fingers do get pointed, point out that it has to be reasonable for you to take a vacation, and clearly if you'd known about the potential for the problem, you'd have done something to prevent it - but no one can see all cases in the future.
Solution - spend only enough time on the problem for your boss and any other stakeholders (other people responsible for the solution) to see the risk imposed by the problem. Move quickly onto the solution (secondary process), and directly connect to how it addresses the problem. Be open to the input of others - the process may need tweaking, since ideally it is not what you do but something anyone can do in your absence - so it has to work for everyone.
Keep everyone focused on the solution. At the very end, verify with everyone that the solution is likely to be a fix for the problem.
Then let it drop.