Lyman;
Re, "I have heard (anecdotally) of the bus abandoning the Flight Path in favor of a turn, caught by pilots who then fly manually to destination."
"Flight Path" is defined as the vertical element while "turn" is obviously the lateral element. VNAV requires LNAV because the descent is defined in terms of altitude and possibly speed constraints which are either part of the STAR arrival or merely slow-down or speed control points such as found at Heathrow or Frankfurt. Put another way, the programmed track between waypoints is necessary before the vertical element (VNAV) can manage the descent to achieve the required constraints.
If radar-vectored off the course programmed, the lateral/vertical elements are no longer managed and become instead selected, (short-term operation), and HDG is selected, the lateral element is removed and the vertical element is logically no longer capable of managing VNAV calculations due absence of waypoint constraints, therefore the vertical reverts to Vertical Speed, or more rarely, FPA, neither of which are 100% suitable but for unimportant reasons. The PF can then choose a more suitable mode such as Open Descent - the Alt Sel on the MCP is reset to the cleared altitude if it was previously set to the lowest altitude on the STAR...some airlines allow this, some don't, "just in case."
The Airbus doesn't abandon anything even if in VNAV it is going to miss a constraint - in such a case it will signal on the PFD the need for speedbrakes.
In short, the scenario you're describing can't logically occur and there are no Tech Bulletins extant describing such odd behaviour so I doubt it occurred at all. Leaving the flight path does leave the FMC-programmed LNAV course and the autoflight lateral mode drops to HDG but that is completely normal behaviour for all these autoflight systems, with minor variations on the theme. If what is meant by "fly manually to destination" is the use of HDG on the MCP and Open Descent then this is the same thing as saying one had to use the steering wheel in one's car to get to one's destination and we all know how risky that is. If one selects a downstream waypoint to which one wishes to "go direct", one selects the waypoint, puts it into the requisite line and the airplane will turn towards the waypoint using the shortest route, left or right. In the Boeing, after selecting and placing the waypoint into the requisite line, one then must execute the selection, leaving the crew one more step to make sure that that is what they want. There is no "maybe" about this.