I am guessing this problem is limited to regional airports and/or domestic routes.
I've consistently had no problem booking VN flights with DL using SM, such as in flights from Europe or Africa to Asia, Asia to Australia, etc. (Wholly or partly with VN long-haul flights. E.g. KIX-SGN/HAN-SYD, FRA-SGN/HAN-ICN, JNB-CDG(AF)-HAN-HND and so on.)