During the RER A disruptions in July and August, your workaround states to take Metro line 1 to Vincennes and transfer back to the RER A in the direction of Marne la Vallee? Which is the specific transfer station? Both Nation and Vincennes will be closed.
Vincennes RER / Metro station will be the transfer station.
RATP Works and Strikes page details how the RER A stations will remain open during construction:
Pendant les périodes et heures de fermetures, les gares d’Auber et Vincennes deviendront des terminus provisoires (ces deux gares resteront ouvertes et les correspondances** seront maintenues).
Translated into English, the bolded text says the two stations will remain open. Auber / Vincennes will be provisional terminus stations, so you'll be able to take Metro line 1 to Vincennes and transfer to RER A from Vincennes to Marne La Vallée.