Horizons has calculated trajectories for deep space spacecraft, including six objects related to Apollo 8 through 12.
See an (old) list in the currently unanswered question What was Chang'e-2's 3D orbit in space? (since it's not in Horizons) ...yet ;-)
But there's nothing there that I can see for the Viking 1 and 2 missions which came later and were certainly well-tracked.
My guess is that there are raw tracking data and that due to time and resource constraints and probably not much need, this is simply lower priority than moving forward.
However, there might be a more interesting story, so I thought I'd ask.
It would come in handy for addressing (in Stack Exchange, not necessarily in reality) Is the 1970's Viking Orbiter due to contaminate Mars?