I would like to see some information from certificates, but as you say the signatures of Robert Dunn on the Will/Probate docuents appear to be the same.
Marriage
DUNN Janet born Ballarat
VIPPOND Richard William born Cape Town
1877 Reg#4496
Births
VIPPOND Janet Elizabeth
Father Richard William Mother Janet DUNN
At Ballarat 1878 Reg#20505
VIPPOND Ellen Margaret
Father Richard Mother Janet DUNN
At Ballarat 1880 Reg#13340
VIPPOND Catherine Maud
Father Richard William Mother Janet DUNN
At Richmond 1882 Reg#25161
VIPPOND Ernest Richard
Father Richard William Mother Janet DUNN
At Richmond 1892 Reg# 6281
Death
VIPPOND Richard William
Father Vippond Richd Mother Elizabeth BARNARD
At Richmond 83 years 1939 Reg#722
The VIPPOND family lived at 119 Somerset Road, Richmond during the period 1900-1936. Richard William was a jeweller.
http://www.fcmp.com.au/Fawkner Memorial Park wishes to advise that the location of the buried remains of the late Richard Vippond aged 83 are located at Coburg Cemetery PRESBYTERIAN: Compartment: D Grave No: 526. and buried in this location -
VIPPOND Ellen Margaret Service date 20 May1946
VIPPOND Richard Service date 14 May 1920*
VIPPOND Richard Service date 28 Jan 1939
* I can find no death for a Richard in 1920 - ? Transcription error from burial register. Could this be Janet who died in 1920. You could enquire at Fawkner.
Cando