It's kind of scenario-specific and I'd suggest agree up front if you or your opponent have or there is any doubt, and create your own 'House Rules'.
Most players get to PBEM after playing hotseat/PO games fairly extensively, if only to learn how to play. Once you've played 2WiN- say- 5+ times you know what events may/may not occur, what the reinforcement schedules are etc... So personally I don't see any problem in looking over the scenario in the editor or via any 3rd Party add-on (there are some or you can DiY) to see what's going (to go) on- events, probabilities, reinforcements etc... and Supply Points. And it makes sense to turn the supply net on to see where your sources and/or Supply Depots are, and to assume your opponent has too, either because he's interested from a play or design perspective, or he's played the scenario multiple times as both sides and has- deliberately or unconsciously- memorized all this information. Consider it valuable intelligence, which both sides are entitled to and can be better/worse at deducing impacts or using to their advantage/disadvantage. Like real life.
But again it's scenario-specific. In Nghia Lo 1951 there is also a French Supply Point on-map, which the VM could just as easily take to cripple French supply before the paras arrive. The French can equally drop on/near VM Supply Points. The designer specifically mentions the French had previously targeted VM supply, so perhaps this tactic might have been expected. Perhaps the designer should have made the Supply Points 'Exclusion Zones' or specifically stated that, as a 'House Rule', neither side's SPs could be approached. It does seem difficult- if not impossible- for the VM to protect their SPs AND seize the valley, but maybe that's what this scenario is designed to illustrate, but I'm just guessing.
Maybe to avoid either party feeling aggrieved, just agree on House Rules and restart or replay or mirror the game. At least playing French in a mirror game you'll get to stomp all over the VM Supply Net too and see how they like it.
Tactics are based on Weapons... Strategy on Movement... and Movement on Supply.
(J. F. C. Fuller 1878-1966)