B767 Captain reported aircraft approached a stall after takeoff due to autoflight malfunction. Flight crew disengaged autopilot and recovered.

Narrative

I was Pilot Monitoring on Aircraft X on Day 0. My FO was newer to the Company after getting off of OE (Operating Experience) a few months prior. The FO took off normally and made appropriate calls of heading select and VNAV at 400 feet and 1000 feet respectively. The FO also asked for the autopilot at 1000. Unfortunately; the VNAV did not engage properly; and the aircraft went into Vertical Speed mode. I was in the middle of a radio call when I noticed the problem. I quickly finished the radio call and told the FO to disengage the autopilot to [get] us out of a stall. He did as I requested. Once we had better control of the airspeed; I had to reset the flight directors prior to reengaging the autopilot. Once reengaged; the autopilot acted normally; and the flight continued. I emphasized to him to never be afraid to disengage the autopilot if it does something unexpected. Since the autopilot reengaged normally; I felt that there was really nothing to write in the logbook on the event. Despite that I did inform the mechanic that met us at the destination about the event in case there was something that he could do.Suggestions: While better stressing the importance of deautomation as well as a better understanding of the VNAV system out of training would be nice; both of these things really come with experience. For this scenario we did get through it; and the aircraft was resueable however; stressing that we can't become complacent on the technology that we have would help newer FO's get a better understanding of the aircraft.