I suspect in practise the train manager might accept an "open jaw" return journey.
That's only because both are part of the cluster of Plymouth stations. If you were going further, there'd only be one fare from all of them. But that's a fares cluster, not a group ... so I don't think it makes the ticket valid at any station, does it?
I was suggesting that the train manager would use his discretion and let you travel out to Ferry Road even if you had a return ticket from Victoria Road, and vice versa - even though it's not really valid. Can't guarantee that, of course, but
GWR▸ would look like silly billies taking someone to court for fare evasion just because they went back to the wrong station. "It's across the road from one station to the other, M'lud".
Indeed. But what officially is allowed?
If you get a ticket from thereabouts to Reading, the destination is "Reading Stations" - Reading and Reading West. That is also Reading routeing group. The same fare applies to Tilehurst, as it's also part of the fares cluster.
At the Plymouth end, there is a Plymouth routeing group - Devonport (Devon), Dockyard (Devonport), Keyham, Plymouth. The cluster includes those plus Ivybridge, St Germans, St Budeaux Ferry Road, St Budeaux Victoria Road, Saltash.
So presumably you never get a ticket to or from Plymouth Stations, in which all members would be valid start or finish points. However, if you can justify going through another routeing group station as part of a normal* routeing process, you can start or finish there (if the ticket type permits it).
*probably not the best word