I can confirm what Jan said above. During the 737-300 heyday, I talked to a lot of 737 pilots in airports while waiting for flights and I asked many of them about this specific thing, as I was struggling with this with the early PMDG. Almost all of them said they rarely used Vnav, primarily because it was not very reliable and also in the US you rarely fully fly STARS to the end, most often getting vectored off early, put in holds, late descents and other ATC related things which pretty much rendered Vnav not very useful. At least according to those pilots at that time. I have a feeling the same thing is true today.