Ok, yes, some of the more rare species can't be found at Digikey. Lots of my RF parts that I need in designs aren't either, such as power inductors or ferrite cores. And when the handle on a hose bib broke Digikey didn't have one. Unbelievable ... :-)
But seriously, the fact that even Farnell stocks only 69 of the 74VHC161 right now and the fact that major distributors list 12-13 weeks lead time versus 4-6 weeks for a 74HC161 would make me think twice about using it in a design. Now I don't know about this one because I rarely use such stuff but sometimes those are the first clouds on the horizon.