The US government will release the 1940 census to the public in 2012; at that time, you will be able to locate their whereabouts by census record.
There is one other way to find out about this family, however.
Since Joseph D. Vertin passed away before the full implemetation of the social security act, it may not be possible to find a record for him, but there is probably a record for his former wife, as the SS Act was begun in 1936-1937, and she will most likely have a file with them.You can request her folder from SSA and it will cost$27.00 for her original application if you have the number, and $29.00 if you do not have the number.Although this figure is high, it will contain every piece of information about her application.You may also petition them for her complete work record, and ask them to price the photocopy or abstract of that record.It will most likely contain her death certificate as well as other factual things relating to her income and adjustments to her provision.She would have been entitled to a small monthly amount upon her retirement.If she remarried and changed her name, you may be out of luck unless you locate her remarried name.If you can find her SS # then that would be helpful.Do a search for her and see what you can find at familysearch.org - it is free.
Death certificates depend largely upon the knowledge of the reporter, usually a relative.Sometimes they contain little, or even have false facts based upon estimations of age.But they might have the locations of burial, and the funeral society that handled the arrangements.Then, if the Funeral Home has not had a fire, they may still have an archival record for the burial that could lead to finding relatives and obituaries.
By the way, Monrovia is an incporporated city within Los Angeles County, not a separate county.The county west of Los Angeles is San Bernadino County.Los Angeles is confusing because there is the municipality and the county with the same name.Native Californian here.
I am glad you located the family records; now you have a base from which to work.