Once manual fwd kicks in there is no fwd to vmail because sys fwd at sending station is disabled and receiving stations sys fwd doesn't affect forwarded calls (what if the forwarding station had no voice mail?).
A few workarounds:
Build a sys fwd that has destination station as point 1, v mail as point 2, enable when sta is in DND.
Manual fwd to a HG that has only destination station as member, recall times out to "empty" CRA that times out to original mailbox.
If this is common put a Secondary Call Key for originating station on the destination station...call stays at original sta, also rings secondary station, but stays on original station sys fwd path