Depression generally speaking is a chemical imbalance in the brain. Simply put it's often a reduction of the neurotransmitters serotonin, norepinephrine and dopamine. Which is what most anti depressants work to increase. And from everything I've read and researched about it (I have major depression, general anxiety disorder, and PTSD) this stuff often stems from life experience and how your brain deals with it. But sometimes it just happens for who the .... knows what reasons. Mid 50's are when my father started suffering from major depression with no traceable causes. As you age things in your brain change and sometimes these things happen. There's always help though and lots of times medication isn't necessary.
Yes, this is true. Depression seems to be a good mix of biological (genetic), and environmental triggers. We know there is a strong genetic component because there is strong evidence that it runs in families as do most psychiatric disorders. Take a look at the Hemmingways for one example (numerous suicides in that prominent family). Most people will notice the onset in their teens or early 20's. However, it can onset later in life without any prior history.
My only point here is that smoking is strongly correlated with mental health issues (whether depression or other disorders). As I said, 44% of all cigarettes consumed in America are consumed by people who have some sort of mental illness. It's well known that schizophrenics are highly inclined to smoke (I have known several, and they all chain smoked). There's some sort of therapeutic property in tobacco (whether nicotine or not) that appears to perhaps give some minor relief.
So if the OP is feeling more depressed after switching to vaping, it is doubtful that vaping is causing depression. It's more likely that he/she is missing some of the "therapeutic" properties of tobacco smoke. It does appear it is more complicated than just nicotine.
O course, there may be no cause/effect here at all, as it's pretty hard to prove cause and effect in individual cases like this. The best science can usually do is show a correlation over a large sample.